Also, if you have a particular brand or era you're looking for, like "Pre- TREK" era Klein bikes or Cannondale ST's, for example; you can do some research (FB owners' groups are a good place) to find out if they have any sizing idiosyncrasies (run big, run small, etc)
Road bikes usually are fairly close to size, but MTBs, especially early ones can vary wildly from brand to brand