An average cap size is technically a circumference measurement of 21.5".
In spite of anything you may have heard to the contrary, this cap size will fit 90% of female wig wearers. This is a range, so even if you are not exactly 21.5" this is still more than likely the cap size you will need. Most wigs have adjustable straps in the back at the nape. Depending on the brand, this range can be from 21" - 23".
This variation means that you may need to experiment a little to find the brand that uses the head form with the best shape and size to fit your head.