Lizardkid:
I'll make it fast and without many details, cause I'm kinda short on time right now...
A Squad, usually commanded by a Sargeant, comprises 6 to 12, or even 16 men, depending on its specialty. Every Squad can be divided in 2 or more Sections, under the command of a Corporal.
3 to 5 Squad form up a Platoon, usually under the command of a Lieutenant. A typical Platoon consists of one Command Squad, 3 Infantry Squads an a few more special teams, like antitank teams, machigun teams or so... Your tipycal Platoon if form by around some 40 to 60 men.
3 or 4 Platoons form up a Company, usually under the command of a Captain. Your typical Company consists of 2 or 3 Infantry Platoons and 1 or (rarely) more Support Platoons, which normally comprises Light Artillery (Mortar Teams), Scout Teams or so... The Command Team of the First Platoon of your Company is, actually, the Command Squad for the entire Company (that is to say, that Squad is under the command of the Company's Captain...) All in all, your typical Company comprises around 150 to 200 men.
3 to 5 Companies form up a Battallion, usually under the command of a Major. Besides the normal Infantry Companies, your Batallion has Support Companies normally holding more heavy duties, like Artillery (Howitzers or Rocket Launcher Systems), Assault Engineers, Antiair or so... Your typical Batallion comprises something like 600 to 1200 men.(Companies and Batallions are the basic field units of modern warfare tactics...)
And 3 or 4 Batallions form up a Regiment, usually under the command of a Colonel or a Lieutenant Colonel. The Regiment is the base of organization for all previous units. Entire Batallions or even smaller units can be assigned to Regiments according to particular needs, and Special Regiments (Artillery Regiments, Sanity and Medical Regiments, Intelligence Regiments, Scout or Special Forces Regiments, etc.) are usually separated in its smaller parts and reasigned to bigger units in accordance to the tactical or strategical needs of those bigger units. Your typical Regiment is formed by 2000 to 5000 men, including, sometimes, civilian personnel.
Normally, 3 or 4 Regiments of different specialties can form up a Division, under the command of a Lieutenant Colonel or a Brigade General. The Division is the biggest independent unit on the field, and bigger formations are understood as groups of Divisions working togheter. A full fitted Division can comprise 8000 to 12000 men, and sometimes even more...
But, still, if we're on a very big operation or battle, having many Divisions on a wide front, those Divisions are grouped into what is, strictly, an Army. The Army is, then, a group of 2 or 3 Divisions working togheter, yes, but under the command of a high ranking General (Lieutenant General or Major General). The number of men in an Army is very variable, and depends on the number and size of its Divisions and smaller support units.
And, if we found ourselves in a really big battle (like, if we were the German Wehrmacht about to invade Russia) we'll need to place our many Armies separated into even bigger formations, to place them under someone like a really high ranking General or Fieldmarshall (as in the case of the Wehrmacht example). So, we take as many Armies as we need and we create an Army Group...
And, still, there are many other independent or special formations, like a Brigade, which is a fast mobility, Division sized group formed of any kind of units to fullfill a temporary or very precise objective (like, for example, the modern Quick Reaction Forces common all around the world, or the practical german Kampfgruppen of WWII).
Ok, Lizardkid... Not sure if this answers your question, but I hope at least will come in handy when trying to start researching about the matter...
(I'm kind of a war history freak, so I guess I can always try to help on that side... As long as you keep helping me on mapping issues...)
Cheers...
- or maybe other ppl are faster than me... 
