Fleet Admiral: Commands a theater of war(coordinates multiple fleets).
Admiral: Commands a full fleet(several lines).
Vice Admiral: Commands a battle group(4-5 lines).
Rear Admiral: Commands a task force(2-3 lines).
Commodore: Commands a single line of battle.
Captain: Commands a single starship

Army Ranks:

Major General: Commands an army of 4 corps.
Lieutenant General: Commands a corps of 4 brigades.
Brigadier General: Commands a brigade of 4 regiments.
Colonel: Commands a regiment of 4 battalions.
Major: Commands a battalion of 4 companies.
Captain: Commands a company of 4 platoons.
Lieutenant: Commands a platoon of 4 squads.
Sergeant: Commands a squad of 9 troopers.
Corporal: Second-in-command of a squad.