If you take one of your original wheels off and clean it, you should find all the info stamped or cast in to the wheel. The wheel will have its' diameter, width and offset. I don't know what the size is but you might find something like 8j x 16 and then another number. For example ET42 which is the off set. If you search the tinterweb you'll find lots of info about wheel sizes, offsets, tyre choices, changing wheel size etc.
Hope that helps ya.