How is the length at the celebrity wigs measured and how do I know what length to buy?

I want to order a lace front wig from celebrity wigs and I want to know how will they calculate the length of the wig which I want to order? My friend told me that they will measure it from the crown to the hair’s tip to back, where the hair would fall on your back. Do I need to measure it myself for placing the order online and send it to them with my order? What if the wig hair is longer than the length I have given them?