I have a Land Pride heavy duty root & rock grapple. SGC1560 at 820 pounds and 63.5" wide. The grapple is used almost 100% to move large rocks and chunks of my Ponderosa pine trees. I also use the upper jaw for backdragging to level and smooth dirt. I have little to no brush here on my property.
The difference between a 60" grapple or a 66" grapple will only be in the added weight carried by your FEL. As far as work that can or can not be accomplished with one or the other- there will be only the weight difference.