Depends on what kind of grass you have and what the typical height is when mowing. Open fields or lots of trees/obstacles?
Initial mowing on my property saw some grass that was over 9' tall. I mowed it with a
B7800 (22-23hp PTO) and a light duty 5' brush hog. To say that it was slow going is an understatement! I'm pretty determined. Sometimes you gotta do with what you have.
I've still got that tractor and a 5' rotary (though it's a slightly better one now) and it still does duty, but is now second fiddle to my NX5510. I run a 6' brush hog on the NX5510. That combo sees a lot of brush, so it's well matched/suited for that; it also is seeing more grass now, in which case I'm looking at getting a 7' flail, but that won't be used for the season's first cutting because grass height can still push up to nearly 6'. A good take-away here is that over time one can probably expect a change when it comes to what implements, sizes, one uses.
My generalized recommendation would be to not go over 5' unless you're doing something that more closely resembles finish mowing. You'll be able to run the 5' fairly confidently. Not so certain you could feel the same with a 6' one: AND, there's a lot more width sticking out from the tractor; unless you're adept at running a brush hog that might not be a good idea to go real wide- OK if everything is in the open, open fields, but if you've got obstacles (trees and such) then you're going to have to be a lot more careful.
I've purchased ALL my brush hogs used. The Bush Hog
B26 I have for my NX5510 (46hp PTO) was pretty beat up and required a bit of work- paid $800 for it (put about $600 into it); I figure the weight is close to 1k lbs (it was reinforced with plate steel); it works just as well as a new one that costs around $3,500.
If I had nothing but fields and grass I'd be using a flail.