Road Bikes using Letter sizing.
 

Does anyone know the cm or inch size for bikes only listed with a letter size? For instance, Nashbar has a bunch of road frames from GT and Schwinn for sale, they are a few years old, but they are listed as S,M,L, XL... I know my hybrid, which is an XL is 23 inches, but since road bikes are supposed to be bigger, Im wondering if they might be different.

Varies by manufacturer but their website should say the dimensions for each size.

Even if you can find a geometry chart on their website, they can be hard to understand unless your current bike has the exact same frame angles. For a tall person (which I assume you are since your current bike is extra-large), top tube length is much more important than seat tube length. Top tube lengths on road bikes are going to be a lot different from hybrids.