A rough generalisation for British Army Infantry units in WW1 (which also mostly applied to WW2). Numbers of men are approximate at full establishment. When units were undermanned, junior ranks would often fill much more senior roles –

Section (12-15) – Commanded by a Corporal.
Platoon (50-60) – Commanded by a 2nd Lieutenant or Lieutenant.
Company (230-240) – usually 4 Platoons – Commanded by a Major or Captain.
Battalion (1000) – HQ Company, 4 full Companies plus it’s transport – Commanded by a Lieutenant Colonel.
<Each Battalion belonged to a Regiment, and shared badges and uniform, but Battalions functioned independently>
Brigade (4,000 – 7,000) – Around 4-7 Battalions coming from a mixture of Regiments. Commanded by a Brigadier.
Division (16,000 – 18,000) (plus around 5,500 horses) – A full independant fighting unit including 3-4 infantry Brigades, plus it’s own artillery, signals, transport, engineers, cavalry and field ambulance units. Commanded by a Major-General. (Note: a “Cavalry Division” was smaller at around 9000 men + 2500 horses)