My one "modern" bike is a 2006 Specialized Sequoia. I had originally bought a 2004 model, but it was too small to be comfortable. The first one was a "Large" and the more comfortable one was an "Extra Large". I had to get a shorter stem for it and now it is perfect. On traditional bikes I ride a 24 or 25 inch frame and can be reasonably comfortable. I am sure the LBS guy would say I need a 23 inch but that is very uncomfortable.

I think you need to use what you think is comfortable.