here's my $.02
afaik, in 1964 vw used 165's on 4.5\" rims out of the factory. so thats a 6.49\" wide tire on a 4.5\" wide rim.
on the front, alot of acvw guys run 125's, 135's and 145's on 4.5\" rims. so that's 4.92\"s, 5.31\"s, and 5.7\"s of tire on a 4.5\" rim.
on the back, alot of acvw guys like to run 205's on either 5.5\" or 6\" rims. so thats 8.07\"s of tire on a 5.5\" or 6\" rim.
some guys even cram 215's on a 6\" rim. thats 8.46\"s of tire on a 6\" rim.
some like 185's or 195's on 5.5\" or 6\" rims on the back. thats 7.28\"s or 7.68\"s of tire on a 5.5\" or 6\" rim.
these are just combos that seem to work. obviously there are plenty of options.
what is important is making sure you get a good contact patch of tire to road.
for instance, if you were to stuff a 135 on a 6\" rim, it would be stretching the limit and you'd most likely start riding the sidewall and probably have traction issues shortened tire life.
i've seen 135's stuffed on 5.5\" rims before, but i think even that is pushing it and you start to get a little balloon action and it just doesnt look right.
likewise if you stuff a really wide tire on a really narrow rim. the middle of the tire will have the tendency to concave and you'll get odd wear patterns and poor traction.
like 1976bus said, a smaller tire than stock up front will turn your speedometer faster than normal and make you feel like you're going real fast.
and a larger tire in the back will affect rpms and shift points.