I'm running a 7 ft. Rhino SE-7 behind my TC-45D with FEL. The 45 handles the 7 footer well about 75 percent of the time. I mainly mow light grass about 3 to 4 feet tall, blackberry vines etc., nothing really heavy. I cut a field of coastal bermuda last week that was about 30 inches high. The field was used for haying for many years and was in great shape. The grass was very thick and healthy. I wound up only taking a 5 foot bite in about half of the field due to the engine bogging down. It would have been nice to have another 15 or 20 PTO HP. Also keep in mind the mower will extend out beyond the wheels about 6 to 8 inches on each side of the tractor. This is a advantage when trying to cut close to a fence line or a building. This is a disadvantage when trying to get in tight places full of trees. Also the 7 footer will not fit between the fenders of my equipment trailer.
In hindsight I don't know I would buy a 7 footer again. I may go with a medium weight 6 footer like a Bush Hog 286. Of course I could get a TN-60 for the SE-7 !
It all depends on what you want to do with it.