I'd have to say mid-30s, at least for a road bike ridden on CA roads. Sometimes the really big ones can get a little bouncy. Not quite 'dodgeball bouncy', but you know what I mean.

I've got 30s on the '81 Peugeot, 35s on the '71 Raleigh Competiton, and 37s on the '74 Raleigh Sports. The 30s on the PPKN-10 are just that little bit too narrow. Not "Man, these things are too skinny!"... more like, "I wish these were 32s."

--Shannon