I would imagine that you are transporting something else- an implement or two, with the tractor- so knowing that would help.
I have a B7100, and intended to transport my tractor with a tiller and/or a 5' finish mower periodically. I purchased a 5x10 utility trailer, built by the local high school ag shop. It was a feasible way to get a very heavy duty trailer for a very fair price, compared to what they cost at places like Lowe's or TSC. But, I underestimated my needs. With suitcase weights on the front of the tractor, the trailer was too short to haul my tractor with the tiller on. I had to back the tractor on, and hang the tiller over the front of the trailer. I also didn't plan for the finished inside width of the trailer- it was about 2 inches too narrow for my 5' mower. So, I sold the trailer this week, and am shopping for a gooseneck trailer. I have a skid steer now, so the needs are a little different.
Saying all of that, if I were shopping for a bumper pull trailer, I would look for a 6x14. That would give you enough width, and enough length. If you have a loader on the front, 14 foot may not be long enough, if there is also an implement on the rear.
You could check your local high schools or tech schools, to see if they make and sell trailers. In my case, they sell them for the material costs.
You can also shop for a used one- in most cases, a used trailer is fine.
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