Thing is, actual C-to-C measurements along the top tube don't match up with the size tables, at least not with the two sizes of frame I have. Hence my putting some real-world data above.
As the decals already suggested (and the fact that I never saw a '94 fsr in an other colour despite red), it's a '95 frame, I just looked for the frame number
So if the chart of Specialized for 1995 is correct, it's a 18" frame. Whatever, top tube is 56,5cm measured along the tube center to center.