http://www.1010tires.com/tiresizecalculator.asp
This site was what made me go with it.
You input standard tyre size at the top, then put in a couple of different sizes you were thinking of & compare the rolling diameter.
E.g.
Standard is 225/45R17 = 24.97 in 634.23 mm
235/35R18 = 25.02 in...