I bought a "new" 2000 Merlin frame (Fat Beat) today but think I may have misjudged its size. It is clasified as an 18.8 inch. I thought that would be a 'medium' but am now thinking it is closer to 'large'. I am about 5'10" with a 31" inseam. I typically ride mediums in other frames and they fit me well. I had a Giant XTC that was an 18.5 and it fit me OK (hence the purchase of this frame). I put a set of wheels on it to check standover clearance and it seems snug but OK. The top tube appears to be about a half inch longer than my other bikes. I previously owned one of these but I now think it was a 17.5. Unfortunately, I don't remember much about it as I only had it a brief time and it has been a few years ago.

My question is whether others my size ride this frame and/or is it likely too big for me. I would like to know before I build it up as it would save me both money and time. I will likely try to sell this if it is too big. Thanks in advance for any comments.

Of the cannondale headshok models? A merlin with a fatty headshok.

If it is, the stem typically uses negative rise like a road bike. Might have a longer, flatter position. More flat-back, long reach, racer setup. Flip the stem to get more upright.
Also using a zero offset seatpost and a short stem should keep you within reach, and centered on the bike.

Only going from cannondale experience, but hoping it helps.