It's best to choose your dildo first, then choose your harness.
If your dildo is wider than 1.5 inches diameter, you will need a leather (or vegan pleather) harness with an interchangeable o-ring.
Most of our leather harnesses will accomodate ALL widths of dildo - we supply 3 interchangeable O-rings to hold the dildo securely.
Fabric, boxer-style harnesses, such as Rodeo, as well as Sh! Thong harness & Thigh harness all have fixed o-rings that will not take a wide dildo.
When choosing your dildo, pick a slightly longer dildo than you would if you were using it by hand, as at least an inch of the dildo (more if curvier bodies are involved) will be lost, once it's strapped into a harness. The dildo needs to have a flared/wide base to keep it securely in the harness (which all dildos we carry do:).