I used 1x4 poplar for the frame, sandwiched between 1/4" birch ply on both sides, You could also use 1/4" ACX or CDX (if you can find any in this thinkness). Birch was nice since I was 5' wide and it came in that dimention, also it will look nice on the floor when finished. 1/4" was much stronger than I anticipated, and if you plan on insulating I would go this route. If you don't insulate then 1/2" should be plenty. The main thing to think about is how do you plan on attaching your sidewalls. If you frame with 1x4 then this should be plenty and the thickness of the floor itself is only about supporting your weight.
Here's what I did. Good luck!
