I have a 5' Bush Hog brand Squeeler model rotary cut mower. The manual recommends either 48" or 54", I don't remember which. I talked to the dealer and he said he would get me what I wanted, but it would easily handle a 60". I brought my tractor to the dealer and tried the 60" one on it before buying and it was ok. They make a lighter duty I think called a Razorback, but I was concerned about how it would hold up as it was clearly lighter built. The 48" one would have been the same width as the wheels which I would not have wanted and the 54" one's are hard to find although I think Land Pride makes one. The rotary cut mowers are pretty heavy and it sits way back, so I leave the FEL on when using it. It is also my defence against spider webs in the trees.
As John Thomas said, roughness of the ground and how heavy the grass and weeds are affect the speed you can mow at. We had and area that had not been mowed in at least a year and mowed it in low range but usually in mid range as fast as that range goes.
The MMM is very good and if you don't have brush to mow over or rocks to dodge or it is not really out of control, you can mow some pretty heavy grass with it. You can set it to 4" or even raise it higher than that.