TX (Sorry, couldn't resist, since I read MD as "Maryland" instead of "medium")

I would call it a small frame, though I've never measured it. The frame was given to me for free about 3 years ago. Since it's an older model with a level top tube, the size in centimeters won't directly correspond to a newer bike. My road bike is about 21" center to center. (Height of the seat tube, measuring from the center of the bottom bracket to center of top tube) I just measured "Old Red" and she's 18". About right, since mountain bikes are made with higher bottom brackets for off road clearance. My legs are a slightly shorter than average with more of my height in my upper body.

BTW, reach to the bars an comfortable pedaling position are what is important, not standover height. There's an old rule of thumb that says you should have 2" clearance from top tube to crotch when standing over it. I had a bike like that once. WAY too small and could not get comfortable no matter how long a stem I put the handlebars onto.