I went with 18 x 3.5 front with a 130/70/18 Avon & 18 x 4.25 rear with a 150/70/18 Avon.
Definitely make sure you pay attention to the load ratings on the tires as some of the 150s dont have near the carrying capacity of a 140 made by the same manufacturer.
The 21" looks great. I used Avons and the bike rides and handles great. Its got a 21x3.5 front with a 120/70-21 tire. The back is an 18x4.25 with a 150/70-18. The 18 is closer to the stock diameter. Its a tough choice, but I'm happy with my Performance Machine/Avon setup.
Oliver,
what manufacturer did you go with on the rims?
M
I went with HogPro.They have their own line,which is what I bought,and they also carry Pro-One,RC Components,Performance Machine,Weld,Sturgis,and a few others.