A squad is roughly 8 to 12 men
A platoon is 4 squads
A company is 4 platoons
A battalion is 4 companies
A brigade is 4 battalions
A division is 4 brigades (roughly 15,000 men)
An army is 4 divisions
These numbers vary widely. Active combat units tend to be more fully manned than rear echelon units, etc. (This stuff only applies to the Army. Don't know about the other branches.)