Beginners with moderately advanced hair loss usually feel most comfortable with a 17x17 cm cap - it provides enough coverage and has a natural-looking amount of hair.
For advanced hair loss or if you want more hair volume, a 20x20 cm cap is the best choice.
Other cap sizes are available as custom orders, you can usually find these two sizes in the store. For more info on cap sizes, check out this video: bit.ly/3jUMiPE