The ribs are the vertical part of the sides of the violin that separate the top of the fiddle (with the F holes) from the bottom of the fiddle. Whether they want the measurement of the ribs only or the thickness of the violin (including the top and back) I can't say. You'd have to ask them that. Here are the parts of a fiddle.
A positive attitude may not solve all your problems, but it will annoy enough people to make it worth the effort. ~Herm Albright