I think 48" would have been too small, a 54" would probably have been perfect. I imagine 60" will be fine as long as you're not planning to do aggressive digging with the box or make frequent turns while grading.
Remember that the top link has a huge effect on how the box works and how much load it puts on a tractor. You should experiment with a whole range of top link settings to get a feel for a box and truly understand what the tractor can manage. It's all about varying the angle of attack of the rear cutting edges, which determines whether the box will dig/fill or distribute/feather. You can drag a large box when feathered or going straight. The same box can stop the tractor dead in its tracks if the rear cutting edge is set for aggressive digging, or if you attempt to make a turn.