We have an '86 Deere 750 with a 20 hp yanmar diesel. It is rated 18hp at the pto. We use a 4ft Bush Hog Squealer rotary cutter. In 1ft tall weeds, ground speed is limited to2.6 mph in 4th gear. In 5 ft tall brush we have to go down to 3rd gear at 1.6 mph or even 2nd at 1.1 mph in really thick stuff. There is no way I would want a bigger hog on this size tractor.
In addition to power is the issue of weight. A quality 5ft hog weighs a good bit more than a 4ft one. It takes 5 suitcase weights on the front of the tractor to make lifting the 4 ft hog comfortable. If you have a loader, this isn't really a concern.
Opinions will differ though. I know a gentleman that used a 5 ft hog behind his Deere 650 and said it did fine. I didn't inquire further, but I can only assume he was using a very lightly built hog and cutting his land in frequent intervals.