Scuba Choice Scuba Diving Fin Spring Heel Strap with Quick Release Buckles Size Chart: Small - 7.5" long spring; Medium - 8.5" long spring; Large - 9.5" long spring. Fin knob insert measurement (small hole): 14.6mm diameter. Fin knob insert measurement (bigl hole): 20.4mm. Fin knob hole middle measurement: 12.6mm. Fits most fins, including Scubapro Twin Jet Max, Aqua Lung Rocket, Sherwood Triton, Tusa Imprex/Liberator/X-Pert Zoom/Solla and more.