How large can they be safely?

If I'm looking at a 4x8 trailer, is there any reason I can't put a sturdy 6x8 frame on the trailer and then build on top of it. so it's 1' extra on the left and right? Or does the foundation (or whatever it's called in a trailer) have to be exactly the size of the trailer?
How do I know how tall the whole unit can be built (not counting the trailer itself)? Is there any reason I can't make it high enough for my 6' husband to stand comfortably?
Thanks!
Sandra
How do I know how tall the whole unit can be built (not counting the trailer itself)? Is there any reason I can't make it high enough for my 6' husband to stand comfortably?
Thanks!
Sandra