I put a 110x19 on my rocker and it fits real nice to the fender. Do a search on here. A bunch of folks have done this. According to them, if you put a 21 on the front, you can just re-drill the fender mounts on the stock fender and remount it. Good luck.
that's what I run on mine, I recently put wide glide trees on my bike, but not because of the tire being too wide for the mid glide. It filled up the mid-glide front very nice. You will also notice a huge improvement in handling. Definately do it as you will not regret the decision. With a 21 you will have to go with a new fender from Bitchin' seats or heartland as the stock fender will not have the correct radius for the 21.
The bike is lowered 1" in the rear so the front has stock ride height, but the 4 degree rake essentially lowers the front 2" by kicking out the front making the bike level.