This is where forums like this actually help. My case in point was with my favorite road bike. It came to me with severely worn, aged and rotted 28's, which were really crowded, especially in the rear triangle. A little looking around here found a link to some old catalogues, and lo and behold, my bike was designed and built for 25's. In fact, that's what they shipped from the factory with.

My mixte came with some cheap 32mm folders, which were (to put it kindly) incredibly sluggishly hard to pedal with, and extremely mushy in corners at speed. I rode on them less than a day- just long enough to ride it to the LBS and get some higher quality w/o's in 28mm. I haven't looked back since.

-D.S.