Here is the size chart for choosing Watery Calder Pro Neoprene Jammer. For these jammers, your hip measurement is used to find your correct size.
If you happen to be between two sizes, choose the one that is closest or based on your preferred fit. You need to assess which fit you want; for a tight fit, choose the smaller of the two sizes, and conversely, choose the larger of the two sizes if you prefer a looser fit.