I built mine a couple of years ago for my 40'. Decided to make it big enough to take the largest MH on the market after watching a friend of mine have to enlarge his when he bought a larger MH. The incremental cost of going larger pales to insignificance compared to making one larger. Good thing, cause our new one is 44' overall. I do take the tow bar off for ease of walking around.
Ended up with 48 x 20 with a 16' W 14' H door. 6 inch slab. 60A service. Plenty for room for the coach with slides out, workbench, and storage shelves.