You should identify the shape (Oval, Round, Rectangle, or Square) that most resembles the shape of your prescription eyeglasses, and then according to the size to find most suitable sunglasses frame.
You need a ruler and measure your frames for width from one side of lenses to the other side of lenses in inches. Record the data and follow instructions on diagrams in pictures.
Clip-ons for sunglasses come in a variety of shapes, sizes and colors, so you need to measure your glasses frames to get the correct fit. The width, height and bridge height should be measured by ruler.