A European extra-large cap size is a circumference measurement of 23.5". They are only slightly larger than an American large cap size.
This is a rare cap size, and as such, will be the ideal for for less than 4% of female wig wearers. The same general rules about this cap size can also be said of US large cap wigs. 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.
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.