Front stock size is actually 110/90-18. Some peeps have gone for 120/90-18, bigger tire. Some fitted without issues, others had trouble with rubbing the fender.
I think the ones using 100/90-18 did so because they wanted an Avon tire and it only comes in this size.
Fattest on a stock rear rim is 170/80-15. After that you need to start swapping washers to create an offset, otherwise the tire rubs on the swingarm on the left side.
Metzeler ME-880 is expensive but lasts a long time. Shinko T-230 is cheap (half the price of Metz) and really sticky, but wears quicker (I got 7000 florida miles on mine - no burnouts).
I hate the Dunlops, my bike had them when I got it and they were SO slippery. I have a 491 on my rear now (it was free) and I fell off once already. Ok the tire was new, so I should've been more careful..
