In addition to what others said, you can measure the sit bone width. This can be done by sitting on some memory foam (optionally covered with thin metal foil) and measuring the distance between the impressions. The saddle width should be roughly this distance + 2cm. The measurement may depend on how far you lean forward and depend on your flexibility. Repeat the measurement several times and check the consistency. Too narrow saddle will feel like a hatchet which is not possible to get used to because it will press on the soft tissues.