Based on my experience with the one I built here are my thoughts:
As Kidjedi mentioned account for storage/mechanical underneath (if feasible). I had to offset my banquette towards the curb side 4" to account for the depth of the water heater underneath the seating on the street side. Height was close.
Thickness of your materials are accounted for in your storage area. I was using aluminum so it was minimal.
The seat bottom has to account for sitting and the back foam thickness. I'd suggest up to 18" of actual seating depth. I used my dining room chairs as a model and found the seating depth of 14" wasn't comfortable, couldn't slouch, had to sit upright
The height of the seat back was limited due to window ledge (about 8") on sides, 12" on front window. The 8" hits the wrong spot on my back if I lean back.
Square corners may be more practical than curved. If you want to sit sideways and use it for a lounge chair. I curved mine and sometimes wish I could stretch out.
If your table top will lower for a bed then size is dictated for you. I offset that a little by having a pedestal that allows the top to slide side to side and rotate (had to order out of France). See my thread for details.