We can't give you a good answer unless we know what your main uses will be for the grapple. In some cases anything over a 48" is more than you need, and in some you should go with a wider grapple, very rarely do you need one that is as wide as your tractor. There are also pros and cons between grapples with 1 grapple arm, or 2, there are root rake grapples, and flat, open bottomed grapples. Most of the time the flat open bottom grapples are more useful than the root rake (clamshell) type.
Can you give us more info on what you will be doing with it?