I've been told that a general rule of thumb is 5 HP/foot of cutter width. So, 5' = 25 HP, 6' = 30 HP, 7' = 35 HP. I pull a 5' KK rotary cutter with my Mahindra 2615 4WD (23 PTO HP). Works great.
I pull a 6' Howse with 27 PTO HP. Works fine for me unless I let the grass get too high or in one part of my lower pasture where the grass is really thick and heavy. Then I just take smaller bites and/or slow down. In normal pasture I can run in the highest gear at rated PTO RPMs, but I don't really like running that fast.
True that 5 Hp per foot is a general guideline. Be careful as the larger you get the more weight you have hanging out back like a huge lever. Hence your front tires are coming off the ground. This is especially true with the 7 foot cutters. Even thought the general guideline would say 35 HP most mfg. of these cutters recommend 50 HP plus tractors. This is why most cutters over 8 ft wide are dual spindles. The width increases but the lenght does not. For example a 10 ft cutter would only be as long as a 5 ft one.