Frame Size Q
 

I am looking at road/cross frames on eBay. I found a cool looking frame which is stated as a size M, but the only dimensions given for it are 20 inch top tube, and 17 inches from the bottom bracket to the top tube. The mountain bike I ride right now to work has a 15 or 15 1/2 inch frame, and it fits me very well. My question is whether or not this cross frame would fit me alright - I know nothing about the sizing of mountain versus road frames.