As far as size, it depends. As far as mowing, it doesn't take much to pull a rotary cutter (bush hog), but depending on how much acreage you need to mow, you may want a wider cutter if you have lots to cut to speed things up. A 30 to 50 horse tractor will do almost anything. Clearing takes more power, again depending on what you are clearing. But for reasonable sized brush and small trees, the 30 to 50 would be adequate. Hauling with your bucket (say gravel or dirt) is limited by the FEL capacity, but a tractor in this range would lift well over 1000lbs, which should be adequate. Do you need 4 WD? Only if there is lots of mud, but for relative flat pasture land, is probably not necessaary. Now if you do ground engagement activities, such as plowing or disking, go heavier and more powerful and 4 WD.
Personally, I would get an ag type tractor (heavier than a compact) in the 50 hp range, as it will do almost anything you ask of it. As far as brands, all the major manufacturers are good, but I would base it on the strength of dealership in your area. Stick with a reputable dealer with a history of good customer service, as regular maintenance or repairs are important, and you want a dealer with a good service department and maybe even the ability to transport your tractor if you have an issue (assuming you don't have a trailer and appropriate truck). Personally, I am fond of Kubota, but A Deere, Kioti, New Holland all make good tractors. If several are available, sit in them, test drive them if you can and see which feels good. And think about what future use you may not be thinking of. I would rather err on going a little too big than a little small.
And some will say to get a smaller CUT, as it would work, but I like the increased weight of the ag type (say a Kubota M series) I am not too keen on bells and whistles, but just like a good solid no frills tractor. I have a M 6800 Kubota, and there is not much this tractor cannot do.