Queen size with a extra-firm futon mattress (I gave the one that came with the bed away; they are inevitably junk). The futon I bought right from the factory, and cost more than the bed that holds it.
I like a firm mattress, and this is firmer than any coil mattress, and more comfortable than foam which has heat retention issues for me at least. I like a good foam chair or car or bike seat, but not a mattress (I was building motorcycle seats with these foams in the 1980's, when the term "memory foam" didn't exist; you had to buy it from aircraft suppliers. There weren't even any commercial bike seat companies that offered it; they wised up a decade later and called themselves innovators).
Interestingly, futons are built the same way as a Constrained Layer Damping panel .... dense cotton fibre matt, high density foam, then dense cotton fibres again. The difference is it's too thick for a proper CLD panel (where the outer panels in particular should be as thin as possible, while maintaining rigidity) but if you scaled the futon's 8" thickness down to a CLD panels's perhaps 0.75" it's a nearly identical model.