From: Southern California, U.S.A.
A fire team is typically 4 soldiers.
2-3 Fire teams + squad leader for SQUAD (9-13 soldiers)
2-3 (sometimes more) SQUADS per PLATOON (18-39 soldiers + platoon leader + possible support - you can have about 20-50 men)
2 or more platoons per COMPANY (about 100-250 men)
2 or more companies per BATTALION (about 400-1000 men)
2 or more battalions per REGIMENT (about 1000-2000 men)
Keep in mind that starting at company level most armies (especially modern armies) will incorporate support weapons such as mortars, AT guns, AA guns, and perhaps some light AFV's. Depending on the unit there may also be some armor/tanks units, transportation/trucks, signals and HW staff, Military Police units, etc.
When looking at force levels you need to also estimate their overall supply or other levels. So perhaps you can give less squads to a Taliban rebel force unit, while giving some helicopter units or artillery to a USA/RUSSIAN battalion, etc.
At the end of the day, you must also balance the forces for playability. While this might take away from realistic force sizes it makes playing a lot better.
JUST SAY NO... To Hideous Graphics.