Assuming you are talking about a bush hog for agricultural purposes, the size of tractor you would need depends on a few factors. The first is the size of the bush hog. They come in a range of sizes from five to fifteen feet wide. The second factor is the terrain. If you have rolling hills, then you will need a larger tractor to pull the bush hog through the rougher ground. Finally, consider how much acreage you actually have to clear. If you only have twenty acres of flat land, then a smaller tractor should suffice.