Hi, How are ya? I just noticed the message, sorry for the delay.
I went with 48" Wide x 24" high x 5' long. I tried to account for all things I knew I wanted inside, like storage totes. The frig, with the rack I built for it just fits perfectly, with the 1/2 spacer under so it did not scrape the tail gate. Most measurements were to conserve plywood. Be sure to use a good 2 part epoxy wood sealer, soak it! then a good exterior paint, I sealed all panels with silicone after bolt up, keeping it waterproof is the key.
A PWC trailer should be a good base to start with.
Good luck with your build!