40mm is about as small as I'll go for tires anymore. I have one older Peugeot frame that will fit an actual 32-33mm tire (the stays are pretty tight at about 35mm), but I've upped every other bike I have to the maximum possible, which is usually at least 40mm. My "daily rider", a Redline MTB I've converted to a hybrid, has Continentals that are marked 54mm but inflate to about 50mm dead on (Continental Ride Tours).