My Panaracer Pasela 700 x 28c's measure out at 27.5mm on my 25c Ukai rims. I have bikes with 23c, 25c and 28c tires. At 200 pounds and pushing 50 I tend to jump on the 28c bike for anything more than a few miles of riding.

I just got some Vittoria Rally tubulars in 23c, so we'll see how they fit into the mix.