Roadbikes are typically measured metric. If you go to google.com do a search for "metric conversion" and you will find a site in the list with a calculator that converts any metric measurement to any American measurement, like CM to IN. Very handy, but I'll save you the trouble since I have it bookmarked.
55cm is about 21 1/2in (55 Centimeters equals 21.6535 Inches )