You can pretty much squeeze everything you need into a Type A; just tweak the staterooms a bit (10 total, with double occupancy and no low) and carry about 20dt of small craft (a Firefly's launches are more in the neighborhood of 10 dtons each). You end up with about 70dt of usable cargo (not including the air/raft "mule"), which roughly corresponds to the main bay if I'm reading the deckplans right.
In fact, not much squeezing is really necessary. Just a little, maybe:
http://eaglestone.pocketempires.com/ships/Serenity-25-percent.png