One of the Fiber Stream
owners might be able to comment on the room available on both the back and front sides.
Although I don't know the specific reasons in this case, I understand that there would be multiple reasons to get away from the original tire size/type, which I understand to be 4.80-12. The thread Fiber Stream Wheel, Bearing Sizes
discusses both a swap to 13", and the more common slight upsize to 5.30-12.
Personally, my impression is that any available 13" ST tire would be a questionable fit, and I wouldn't want to raise a trailer of mine just to fit bigger tires
. In that earlier thread Frederick mentioned that Chris had swapped to 13" tires, but the exact size is unknown.
I have long thought that the logical upgrade from those old 4.80-12 and 5.30-12 tires is the 145R12
(presumably specifically ST145R12, LT145R12, or 145R12 commercial). These are tires of much more modern construction (say, 1970's technology instead of 1930's), not much larger in any dimension than the old stuff, requiring substantially less air pressure for the same load capacity. A 145R12 would fit on the same 12"x4" wheel as a 4.80-12 or 5.30-12.