I run a 6'-0" with a 40-45 hp tractor, in heavy grass, going up hill it is about right, more hp would help there as it's about as much as the tractor will handle in 2nd gear, drop to 1st gear, easier on the motor, maybe a 7'-0" cutter would still work in these conditions 4'-0" - 5'-0" orchard grass. Thinner or not letting the grass fields get so high is definitely easier on the tractor, you can use higher speeds, but the same fields will be cut more often.
One thing that helps is having good cutter blades, just like a lawn mower, cutter blades should be balanced, blades replaced if worn, bent or can't be sharpened, and balanced out, you will lose some performance, which can make the difference if the hp is marginal for the cutter and conditions you are mowing. I'm sure a lot of people may not check the weight of each when sharpening, but when new or in reasonably balanced condition you don't get vibration, it is hard on the bearings in the gear box, you'll really notice it when first engaging the pto, I've sharpened new blades the first few times after installed while on the mower, taking even amounts off with the grinder, still can get some start up vibration, but goes away immediately, new blades don't seem to do this, so when possible not bad practice to weigh em, balance same.
Seems a lot of little things can change performance, you want to make sure the deck is level side to side and the cutter is adjusted to the tractor according to the manual on it, most tilt the front of the deck down a bit, rear higher, adjust the tail wheel, top link, use the level box for side to side levelling, that and sharp blades makes a difference when near or at the max width for the tractor.
If tall grasses is the worst of what you cut, 6'-0" will be good, heavy brush, saplings can be real nasty work, might want to consider other options for those jobs, heavy grasses, weeds better to cut high sometimes, then lower the deck for a 2nd pass, will neaten up the job, I sometimes go back over the field if I let it get too high to clean it up, shreds the cut grass again etc.