Its only two models for the FX's; the FX1 and FX1 PRO. These are the FX1 PRO's which are 17.5" across the waist(36 inch waist line).

This is from there website concerning the FX1's:

The FX1 Pro racing seat features composite fiberglass technology. The FX1 Pro racing seat comes in three different sizes, the FX1 Pro, FX1, and FX1 Wide. The FX1 Pro combines comfort, performance, and safety at an absolutely unbeatable price. Designed with an emphasis on comfort, this seat is one of the most comfortable race seats you will ever sit in. If you enjoy racing and want a practical and affordable shell seat, the FX1 Pro is a hard one to pass on. As a rule of thumb, the FX1 Pro racing seat will fit up to a 36 inch waist, the FX1 will fit up to a 38 inch waist and the FX1 Wide (black only) will fit up to a 42 inch waist. High wear patches are strategically placed to protect your seat from abuse in the high wear areas. The FX1 Pro racing seat is available in cloth fabric with leatherette high wear patches or black microsuede with leather high wear patches.