The size can be anything you want but we went with a standard twin size mattress. 75x39 . HOWEVER ! side note dont make this taller than your widest door way !! our day bed will forever live in this room till dismantled lol!
Step 2: Sheet Em Up !
Cut squares out of 1/4 inch ply. I used a pneumatic stapler and glue. My son helping in this last one ..