Tire size calculator:
http://m.discounttire.com/dtcs/infoTireMath.dos
This will give you approximate, not actuals.
Also for any tire size/brand you are considering, Tirerack.com has all the specs such as reccomended rim width (but not the sidewall height) listed. Go to the site, search tires by size, enter one size you are considering and click the specs tab for other sizes/specs.
Good luck!