You can set it in MED yourself. The model can be dead simple. Four vertices and two faces are sufficient for this purpose. Just move the four vertices to appropriate coordinates.

If you want to find out by code you can create a single temporary entity with the corresponding file, use c_updatehull and then read the value. The width should correspond to ent.max_x - ent.min_x. ent.max_y - ent.min_y should give you the same value if your entity is really quadratic.

http://www.conitec.net/beta/ac_updatehull.htm