A petite-average cap size is between 21" and 21.5". Due to the adjustable straps in the back of most wigs, this will be a comfortable wig size for most petite and average cap size wearers. This size will work for about 65% - 70% of female wig wearers.
This cap size is most typically seen in medical wigs for Alopecia Universalis (all over hair loss) or profound loss on the scalp, as can be seen in people undergoing chemotherapy. The caps are generally made a little smaller to provide a secure fit for clients who lack biohair under their wigs.
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.