How much clearance do you have now? My 550 I had a 110/90-19 (which is I think about 3.5") on it when I got it (on a 2.15" rim, not the original 1.85" rim). Fender size isn't the only factor, rim width matters too. 3.25 is probably already at the limit for the stock rim, 3.50 or 110/90 is probably the max that comfortably fits under the fender.