Adrian,
Not sure if largest wheel is the right thing to look at. The tire size will determine the space needed. Sure, the wheel has to fit, but the sidewalls and tread will be the main concern. I like to go to
http://www.tirerack.com/tires/tirebran.jsp to check out the section width of a particular tire. You can also see which size rims are recommended for which size tire. Pretty usefull.
I have a Karmann Ghia and we all know how limiting those finder wells are.
Take care and good luck in choosing the right wheel/tire combo. BTW, backspacing is very important also as you already know.
Kelly (Cohibra)