I've got a cheap A-Bike knock-off with 6" wheels. It's ride-able on smooth surfaces, but you feel every crack in the pavement. Gravel roads or other loose pavement are out of the question.

On the other hand, my Strida with 16" wheels is fine on gravel and every other road surface I've come across.

Between those ranges, what is the smallest PRACTICAL wheel size for typical city use (mostly paved, but with the occasional rough patch)? Is 6" even usable with perhaps a wider tire or some kind of suspension?

Looking for replies from people who actually own and ride on smaller wheels.