A large cap size has a circumference measurement of 23". If your circumference exceeds 24", then you should only consider large capped wigs.
This cap size will be the ideal fit for approximately 4% of female wig wearers. Most women who think they need large cap wigs should actually consider average wigs instead.
At CysterWigs, we recommend avoiding large cap wigs unless you need additional room ALL THROUGHOUT THE CAP (not just the perimeter), and cannot find an average-sized cap head form that will fit you properly. Large caps not only have additional material at the perimeter, but they also have additional material built into the rest of the cap, particularly at the crown. This can lead to fit problems -- bunching, slippage, et cetera -- due to the fact that a lot of people are average capped throughout most of their head and just a little wider at the circumference. (Remember: the adjusters on most average wigs will allow you to let them out to accommodate a circumference of about 23" -- and sometimes a little more -- for most brands.)
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.