I think a 36 inch rotary cutter would be a waste of time & money..... I have SCUT (19 hp at PTO) and think a 54 inch is probably pretty close to smallest size I would consider and its only 6 inches wider than my 48 inch deck on my JD riding mower...
You can run a 48" unit behind your tractor. My neighbor did so for several years. It does push it to the max, but works. I think a 42' would be perfect.