Originally Posted by BluesDawg

To the extent that sizing convention even exist anymore, MTB sizing is typically in inches and road bike sizing is typically in centimeters and refer to the length of the seat tube, sometimes actual, sometimes virtual, with MTBs typically being shorter. 17' MTB and 54cm road would both be considered a "medium" sized bike. With all the variation in sizing conventions, I usually look at effective top tube length first when choosing a size.