bike sizing

Question
Hi-
I currently have a Giant (recreational) mountain bike, but I find that I'm riding more on paved bike trails than dirt trails, so I want to get a road bike.  My mtn bike is size large, which I think fits me OK, although the bike shop I first saw this bike at recommended a medium for me.  I'm 5'11" and medium build.
I'm looking at used bikes on eBay because I can't spend $1000 or more for a new bike right now.  How can I know what size bike I should be looking at without going to a bike shop to get sized for a bike I won't purchase there?

Answer
Hey,

You can still go the bike shop to get fitted. Thats what I would do. Even if you dont buy a bike from them you will get expert advice and also when it comes to biking, most shops dont mind giving info and helping out fellow riders.

Also if your 5'11 I would say you need a medium size frame also. Another thing, why dont you look into a hybrid bike?

~ Thanks and