Actually I'm not quite sure of the size - it's quite possibly smaller than an 18", it's yellow and has a small dent in the center of the the top tube on the non drive side. You can see it in the profile pic. If you zoom in there's a dot right in the middle where the paint has chipped (it almost lines up with the seat post in the background).
It's been sitting brand new for 20 years and somewhere along the line it fell or got knocked over...so there you go.
I'll pm you the shop info