I've never been real good with fitting wheels and tires as if I wanted a larger tire, I would just lift it (I'm more of a truck/jeep guy lol). Here is what I would do (and have done with pretty good results).

Measure the tire height that you have right now. Then look for a sticker (usually in the door jam) and see what size came stock. For the front tires, I would try to keep the tires the same width (maybe move up one size I.E. - 215/65/15 to a 225/60/15) so that way you do not rub at full turn. This is also going to be dependant on the back spacing of the wheels you currently have verse what you will be getting. If your new wheels will have less backspacing, then you can go with a little wider tire (I.E. - 205/75/15 to a 225/65/15). I would just try to keep the height as close to stock as possible.

The rear will be about the same minus the turning factor. You Will need to compress the suspension as much as possible to see what height tire you can go with but I'd be willing to bet that you can get away with something allong the lines of a 245 or 255 wide tire about 25" tall.

Looks like the Duster could use a set of leaf springs in the back. What does it have for a motor. I think a little wider tire in the back would look killer.

Please check out my albums, any ideas on how to make the Duster cool, let me know!!!