SOIL CONDITIONS will be your big factor.
If you have lots of clay, then size it down. If you have an old farm field that was worked yearly, then you shouldn't have any tree roots, etc. If you have sand, then you have no problems. If you have lots trees, scrub, bushes, then you will have lots of roots and may have to size it down.
I think your tractor is a 21hp? If so, you would probably want to stick with 48" to 54".