What are you going to pull it with?
When I built mine, I have a 5300 JD 50 hp, I built it 18 ft and as wide as the bed on my 1ton. I had built some earlier 8ft wide, but when backing, being that the trailer was wider than the truck it seemed overkill and unhandy. Also when going through narrow gates etc. had to be cautious. This one is 7'6" and I like it much better. If I build onother one though I will make it 20' long. It will be better to load onto and with the brush hog on it won't hang over so far.
I installed two axles at first, but had my small dozer on and lost a wheel so went to three. For just the tractor two would be enough. Mine tilts up to load and is very easy to fabricate this way. Is nice not to have to pack around ramps and such.
If you would like some pics of my design would be no problem.