Division: The largest 'unit' available. It's a structure, or a mobile command base if you want to look at it like that, with some defense and not much else.
Brigade: 4 brigades per division, this is also a structure giving some defense and nothing else.
Battalion: A "normal" unit, use your imagination as to how many actual soldiers are included. At low tech, you can imagine that it's 500 soldiers with pistols. At high tech you can imagine that it's one walking assault mech the size of a building. You can roleplay/imagine what you want here. You can assign 4 battalions to a single brigade.
Company: 1/5 the size of a battalion, in case you just need a marine company to capture a fast enemy ship and the company drop module weighs less. Or if you just need a garrison company to quell some unrest, or somesuch.
A battalion, brigade HQ, and division HQ all occupy one regular sized drop pod and troop bay. You can pack 1 division HQ, 4 brigade HQs, and 16 battalions, forming the entire "group" of infantry over 21 troop transports. Any commander assigned to a brigade or division will share their bonus with underlings, so you can stack up your combat bonus commanders and end up with some fierce heavy assault troops.
Engineering battalions are 5x the size and are probably more akin to the peace corps than the army, trained to construct in the field but not how to fight.