I have a 4x8 trailer. 42" and 20" mowers.
Owning a 4x8 I would skip a 5x8 or 5x10 and go to a 6x12 or 7x12.
You won't be able to fit a 60" mower on a 5' wide trailer and if the mower has a bagger or trac vac there is a good chance the mower won't fit on a 6' wide trailer.
You may not be ready to move up to a bigger mower now but who knows in a year our two. You didn't own that trailer long enough for it to pay for itself.
I have done a few clean up's where going from a 4x8 to a 5x8/10 will still of had me make more then one trip to the dump.