I have a much older B7100, with a new rebuilt motor, and I have run a Landpride RTA 1550, which is a 50 inch tiller. The tractor ran fine, I had no problem with the tractor handling. It does get light upfront, so you will need some front weights or a loader. I also suggest a quick attach as it will move the tiller back 4 to 6 inches and it gives you a little more room with the PTO shaft to hook up and operate.
Now I bought a used Caroni FL 1400, it is a 56 inch tiller. The tractor doesn't have any problems with this tiller either, power wise. My biggest issue is that the tiller really digs in and goes deep and I have to pull it up or it will bog the tractor down, and it will start blowing black smoke.
All the ground I am tilling has been previously broke and is rock free. If I do break new ground, I mow off all the grass as close as I can. Then I rip the ground and break it up with this homemade tool. I change the cultivator shoes off and replace them with ripper points.
I wouldn't go any bigger than 56 inches, 48-50inches is ideal as it will cover your wheel tracts.