I've got a TC33D that I use on 80 acres: mostly wooded, but probably 15+ acres of "pasture" (at least it's starting to look like pasture). I bought the TC33D because I couldn't go much bigger and still get down some of the trails I had in the woods. If I were just doing open pasture without a lot of obstacles, I'd have gone up to the next frame size larger (TC35 or TC40, probably).
If your pasture is largely open, you could do it with the JD 790 (or a New Holland TC30 for example) which will handle a 5 ft brush mower, but would save yourself some time by going larger say a JD 990 or NH TC40 and a 6 ft brush mower. (some one else can supply you with the Kubota equivalent models)
For me, the smaller frame tractor worked fine, since I'm not mowing all of the pasture very often each season. In fact, in one of the pastures I may end up only mowing 1/3 or so of it each year on a rotating basis, to provide better wildlife habitat. (the other pasture is actually my neighbors and is used for horses, will be mowed as needed throughout each season)
John Mc