Construction Brigades are called that and while they are size 25, ie the size of a brigade HQ + 4 battalions, they have to be attached to a brigade HQ, not divisional HQ. So even a Construction "brigade" would take 25*4+5/5 so 21 troop transport bays, making it the same size as a combat division. A full construction division would be the size of four combat divisions. You probably don't want to create one.
And while 21 Troop Transport Bays is enough to transport a full combat division in one go, I always add 4 more bays so I can move 4 Replacement Battalions with the division, so that it can replace combat losses immediately. It doesn't increase the size of the ship massively and reduces micro-management a bit since you don't have to move REPs separately.
If brigade HQ commanders add their Xenoarchaeology bonus to their brigade, there IS a reason to have Construction Battalions in a brigade. The higher the Xenoarchaeology bonus, the greater chance the installation will not be ruined when you get an attempt at recovery. That would be an example where a small number of units with the best bonus is better than a larger number of units without.
You can find a lot of useful information on ground forces here hxxp: www. aurorawiki. pentarch. org/index. php?title=Ground_forces#Special_Operations_units. As it states, divisions (the two low tech ones, which you probably want to upgrade rather than moving around) have a size of 50, and troop transport bays have a rating of 5 (small troop transport bays have a rating of 1) so you'll need 10 troop transport bays to move one low-tech division.
If you're referring instead to Division HQs, a Division HQ can fit 4 Battalion HQs, which each can command 4 Battalions, for a total of 21 units = 105 size, = 21 troop transport bays at max (not counting construction brigades, which you probably won't want to put in a battalion HQ anyway).