Any full size fills up the space the best. I've used a queen, but it's kinda hard to cram in there. I ended up cutting out my own roll up "mattress" from an RV mattress pad, fits around the wheel wells and such.
I've never had good luck sleeping with an air mattress. Maybe mine was just cheap, but I felt like they always ended up feeling like sleeping on a hammock sideways, turned my back into a banana.