6x12 will work comfortably for a 2 seater or full for a 4 seater. I have a 6X12 I built to haul 2 4 wheelers or RZR. It has an expanded metal floor for traction and dirt/mud just falls thru. Has a rear gate and also ramps for side loading the front 4 wheeler. Large aluminum tool box on the tongue for tiedowns, dirty gear, etc. All LED lighting and spare tire on tongue. 3 years old. $1200
Good info here.
Also, if you plan on pulling your trailer off road, I would recommend a dual axle. They will help get you through the ruts and ditches without dragging bottom.....
as bad.
I had a single axle 6 x 12 built with diamond plated metal floor, square tube railing, and rear gate a few years ago. The floor is a flat 12' and not 10' plus 2' dovetail. Has plenty of room for my pioneer 700 and a generator or anything else I would need to strap down behind it.
6'x12' is what most guys around here have to haul "just" their UTVs. If you factor in front/rear bumpers like some have, then cargo, ice chests, etc, some have 14' even and longer.
Once you decide on size, you need to start looking at build. Joist spacing varies from make and model, outer shell varies (0.024 or 0.3), plywood wall and floor thickness. These and more add into price. Budget trailers are just that.