Most of the knobbies you find aren't going to be in a 17".
You can run a 120/60 or a 120/70 up front, and I'd be willing to bet you can even squeeze a 110/70 on the front.
As far as the rear, you can run up to a 160 width rear tire, but you can also run a 150/60 or 150/70. Remember, if you run a smaller width tire or a smaller tire in height (150/60, as an example), you will lose rear ride height which will change the way the bike handles. You may like it, you may not.