How big a unit is it the HQ for? For a company, probably just a mid-sized tracked vehicle with a bunch of armour, ECM, HQ facilities, comms equipment, and a few tons of personnel space. Maybe C3 masters if the company uses them. For a battalion, you'll probably need multiple vehicles, and they'll be expected to stay a bit further from the front lines, so you can break that up a bit - one for the staff, one for the comms, one for the facilities, and so on. At that level, stealth and battle avoidance starts to look more appealing than ability to take hits or contribute to the fight, so you might even go to light hovers at this point. Same sort of considerations at the regimental level, though by then a DropShip might be the better choice. By the time you get to a RCT or division, you start to look longingly at kit like portable HPGs, and consider moving your HQ up to a WarShip or the like.