Just like a strap-on, your RodeoH harness should fit uber snug in order to stabilize and control the dildo. For help with sizing, refer to the "Garment Specification Chart" on the product page under "View Detailed Images" of the item you are interested in. Each chart has actual garment specifications. We've made this super simple, all you have to do is line up your favorite pair of snug fitting underwear with the Red & Blue line under each style to find the perfect fit. Keep in mind, there is only 1 inch difference in sizing, but that 1 inch can make all the difference in the world. Remember, each style has a unique garment chart. You can also "Click Here" to view our Sizing Video.