Insert your dive boot into your fin pocket, measure the distance from the center of one strap mount pin on the side of the fin, around your boot heel.
To the center of the pin on the other side of the fin.Select the spring strap size that is shorter than your measurement by 0.5" to 1.5" (1.3 to 3.8 cm.
Quick Release Buckles attached.Fin knob insert measurement (small hole): 14.6mm diameter. Fin knob insert measurement (big hole): 20.4mm. Fin knob hole middle measurement: 12.6mm.
Spring length only: Small - 7." ; Medium - 8" ; Large - 9.5". Center of the buckle to buckle length: Small: 11", Medium: 12", Large: 13".