Squad = 6-10 soldiers
Platoon = 3-4 squads or 18-50 soldiers
Company = 3-4 platoons or 60-200 soldiers
Battalion = 3-5 companies or 100-1k soldiers
Brigade/Regiment = 3-5 battalions or 2-5k soldiers
Division = 3 brigades or 10-15k soldiers
Corps = 2+ divisions or 20-45k soldiers
Field army = 50k+ soldiers