The most critical factor for safety is the width of the stirrup iron. A stirrup that is too small can trap your foot, while one that is too large can allow your foot to slip through entirely.

Your stirrups should be approximately 1 inch (2.5 cm) wider than your boot. This leaves about a half-inch (or one finger’s width) of clearance on both sides.

Different disciplines and physical needs require specific stirrup designs. How to Choose the Perfect Stirrups? - Mousm

Most adult riders use 4.75-inch stirrups. 2. Choose the Right Type for Your Riding