A standy style trailer with a bed in the rear that goes from wall to wall (like the picture).

My question to you all is what size of frame a 5x8 or a 6x8? and has anyone done this on a 5x8 that was able to put the entire bed width wise? (all 80 inches in length of a full size bed)
I will be building over the wheel wells as to get more bang for the buck as far as room but am needing some help with some previous standy builders experiences! thanks a bunch