you have 6.87 ft^3 gross volume (before port/sub/bracing displacement) assuming the dimensions you gave me are max external.
with that kind of room the only 15's your going to be able to run are small volume 15's, the type R 15, sundowns Z15 and nightshade, treo ssi15, those are the only ones i can think of off the top of my head. You could run 3-4 12's depending on the 12's, a single 18... as far as maximizing cone area, you best bet would be to find a 12 that does well in 1.5 ft^3 ported and run 4 of them, 4 12's have more cone area than a single 18, 2-15's, 4-10's, if you can't run 4 12's, then 3 12's, 1-18, or 4-10's are all pretty close as far as cone area go. and run aero ports, take up less volume then slot ports usually