I spend most of my time on a road bike, but have several buddies who are pretty heavy into Mountain Biking & keep giving me a hard time about getting out & joining them. I see most of miles still being on my road bike, but am trying to find a used bike that I can go out with them on. Something probably off Craigslist or ebay; somewhere I can get a full suspension bike with disc brakes for less than 1200 bucks.

Anyway, as I started my research on mountain bikes, I quickly discovered they are not sized the same way as road bikes. My roadie is 54cm & I have noticed they are in different increments (17.5? 19.5? etc). I know that a Large Cannondale is too big (my old mt. bike is a Cannondale-large & it is comfortable for around town, but just a little too big for riding hard in the mountains.). Also I realize the most popular answer is going to be go to LBS & get measured, but I am trying to save some money & definitely don't want a new bike. I am just looking for an idea of my size so I don't drive 2 hours to look at a bike that is way off the map as far as sizing.

Any suggestions? Thanks in advance.