The below information is intended only as a general guideline in the definition of the general force structure of our various military components, but it will give you a general answer to the size question. 

United States Army
Army 50,000+ soldiers. Typically commanded by a lieutenant general or higher, an army combines two or more corps. A theater army is the ranking Army component in a unified command, and it has operational and support responsibilities that are assigned by the theater commander in chief. The commander in chief and theater army commander may order formation of a field army to direct operations of assigned corps and divisions. An army group plans and directs campaigns in a theater, and is composed of two or more field armies under a designated commander. Army groups have not been employed by the Army since World War II.
Corps 20,000 to 45,000 soldiers. Two to five divisions constitute a corps, which is typically commanded by a lieutenant general. As the deployable level of command required to synchronize and sustain combat operations, the corps provides the framework for multi-national operations.
Division 10,000 to 15,000 soldiers. Usually consisting of three brigade-sized elements and commanded by a major general, divisions are numbered and assigned missions based on their structures. The division performs major tactical operations for the corps and can conduct sustained battles and engagements.
Brigade 3,000 to 5,000 soldiers. A brigade headquarters commands the tactical operations of two to five organic or attached combat battalions. Normally commanded by a colonel with a command sergeant major as senior NCO, brigades are employed on independent or semi-independent operations. Armored, cavalry, ranger and Special Forces units this size are categorized as regiments or groups.
Battalion 300 to 1,000 soldiers. Four to six companies make up a battalion, which is normally commanded by a lieutenant colonel with a command sergeant major as principal NCO assistant. A battalion is capable of independent operations of limited duration and scope. An armored or air cavalry unit of equivalent size is called a squadron.
Company 62 to 190 soldiers. Three to five platoons form a company, which is commanded by a captain with a first sergeant as the commander’s principal NCO assistant. An artillery unit of equivalent size is called a battery, and a comparable armored or air cavalry unit is called a troop.
Platoon 16 to 44 soldiers. A platoon is led by a lieutenant with an NCO as second in command, and consists of two to four squads or sections.
Squad Nine to 10 soldiers. Typically commanded by a sergeant or staff sergeant, a squad or section is the smallest element in Army structure, and its size is dependent on its function.

 

United States Marine Corps
Regiment 5 Battalions to a Regiment - 60.000
Battalion 5 Companies to a Battalion - 1.200
Company 4 Platoons to a Company - 240
Platoon 4 Squads to a Platoon - 60
Squad 3 Fire Teams to a Squad - 15
Fire Team 5 Marines to a Fire Team
FA Units Field Artillery Units are not referred to as a Company; they are called a Battery

 

United States Navy

I have come to the conclusion that it's impossible for anyone who is not in the Navy to easily figure out the structure of the Navy!!  The following information has been graciously provided by LT Smash since he does understand all things Navy.  Thanks again Smash, you're the best!!!!! 

Make sure to visit his blog and thank him!!!
Fleets

The Navy has two main Fleets - The Atlantic Fleet and The Pacific Fleet, abbreviated as LANTFLT and PACFLT. These are commanded by admirals (four stars)
Admirals are often called "flag officers," because when they are aboard ship, they have a personal pennant that flies from the yardarm.
Numbered Fleets: The Navy currently has five of these. The odd-numbered fleets are subordinate to PACFLT, and the evens are subordinate to LANTFLT. Fifth Fleet reports to CENTCOM. They are commanded by vice admirals (three stars)

Second Fleet - in the Atlantic.

Third Fleet - in the Eastern Pacific.

Fifth Fleet - in the Arabian Gulf and Indian Ocean.

Sixth Fleet - in the Mediterranean.

Seventh Fleet - in the Western Pacific.
The Fleets are organized into various Task Forces, according to their mission. These are commanded by rear admirals (one and two stars).
The Task Forces are broken down further, as the mission requires, into Task Groups, Task Units, and Task Elements.
Task Group A Task Group would be an aircraft carrier and all of her escort ships (sometimes informally called a "Battle Group," and often named after the lead ship, such as "The Enterprise Carrier Battle Group" or, "The Essex Amphibious Battle Group"). A Task Group is typically commanded by a rear admiral (one star).
Task Unit A typical Task Unit would be a squadron of cruisers destroyers (DESRON or CRUDES) assigned to protect the carrier, and is usually commanded by a senior captain who is often called "Commodore."
Task Element A Task Element will typically consist of a single ship or squadron of aircraft. The Commanding Officer of a ship is generally called "Captain," although he or she may often only be a Commander (depending on the size of the ship).
Each ship or aircraft squadron is broken down administratively into departments, which can vary in size depending on the type of unit. A department head on a large ship is usually a commander or lieutenant commander, but on a smaller ship lieutenants may also be department heads.
The departments are further broken down into divisions. These are run by the junior officers, usually fresh ensigns or lieutenants j.g. (junior grade).
Each division may contain multiple work centers, which are run by chief petty officers (CPOs). Subordinate to the chiefs are the leading petty officers (LPOs), and the work center supervisors.

 

United States Air Force
Major Command A major command (MAJCOM) is a major subdivision of the Air Force, assigned a major segment of the USAF mission. They possess the full range of staff functions needed to perform required tasks. USAF MAJCOMs consist of two basic types: operational and support.
Numbered Air Force Two or more wings are usually grouped with auxiliary units to form a Numbered Air Force [NAF] - a large striking organization of one major category of air strength. The NAF is the senior war-fighting echelon of the US Air Force. War-fighting NAFs conduct theater aerospace operations with assigned and attached forces and train to perform this role as an integral command and control (C2) element. Not all NAFs maintain this capability. A NAF conducts operations with assigned and attached forces under a command element. When an in-place NAF is tasked to support a Joint Forces Commander, the framework will be the same as an ASETF, but the in-place NAF will retain its NAF designation (e.g., 7 AF).
Wing The basic unit for generating and employing combat capability is the wing, and some very significant changes have taken place at this level. The wing has always been the prime war fighting instrument. The Air Force has three basic types of wings: operational, air base, and specialized mission.
Group The "group," which usually consists of from two to four squadrons and a group headquarters. The group may be both tactical and administrative. All squadrons in a particular group fly the same type of plane. Groups, like squadrons, are referred to by type of plane - heavy bomber group, light bomber group, fighter group, etc. All squadrons of a group train together and the group usually moves and fights as a unit. It is a vital organization in combat operations and is basic in planning.
Squadron The basic fighting unit of the US Air Force is the squadron. A constituted squadron is the basic unit in the Air Force, and is numbered with one, two, or three Arabic numerals. A squadron may be a mission unit or a functional unit, and may vary in size according to responsibility.

The squadron usually consists of two or more flights. The squadron commander gives orders to the flight commanders rather than to the commanders of the individual planes. The squadron is the smallest air force unit that has both tactical and administrative duties. Each squadron includes ground personnel whose duties are to administer and furnish the ground services.

Flight An organizational echelon identified as a flight is often used within the Air Force. For tactical purposes, two or more airplanes form a "flight." AFI 38-101 (1994) describes a numerically designated flight as "the lowest unit level in the Air Force," and points out that flights are used to incorporate smaller elements into an organized unit. Numbered flights are units.

Frequently mistaken for a unit, however, are unnumbered or alphabetically designated flights such as Flight A of a fighter squadron. In combat, a flight usually consists of four or more planes that fly in pairs, trios, or fours. One plane, the flight leader, contains the flight commander who directs the operations of the entire flight. These flights, sometimes known as "alpha" or "functional" flights are organizational shredouts of a squadron and are not units in their own right. They are used simply to distinguish several elements of a squadron that have similar missions.

Party The Air Force specialists are assigned to Army combat maneuver units around the world. On a battlefield, they form a tactical air control party team that plans, requests and directs air strikes against enemy targets in close proximity to friendly forces. A TACP is generally a two-airman team, working in an Army ground unit and directing close air support firepower toward enemy targets on the ground.

Tactical Air Control Parties provide numerous critical functions on the modern battlefield including: (1) Advising ground forces on aircraft employment and capabilities; (2) Coordinating and controlling aerospace operations and (3) Participating in battle planning.