I would go larger than 5' wide because the discharge chute will add some width, plus the mower might be a few inches wider than 60. Maybe get a 6x8?
60 inches equal 5 feet so there's a possible clue. Really depends on what you plan to do. As in what else you will you haul around and how often. If you have a place to store it I'd recommend a 6x12. I have a 5x10 and wish it was bigger.
If your mower is a sixty inch mower you'll want nothing smaller than a 6x12, most 6x12 trailers will measure about 76 to to 79 inches wide so it would give you the necessary room for the Exmark. If you need a single or duel wheel trailer would depend on what your going to be doing with it or how big a load weight wise you carry. Most single axle trailers are rated to carry 3500 or so lbs.