Purchasing the right size wig cap is essential to wearing a comfortable wig. A wig that is too big may move around, bother your ears and potentially look very bulky. A wig that is too small may ride up the back, slide backwards giving you a large forehead, or even slide all the way off. Either one could cause issues.
Most wigs come with adjustable straps at the back of the wig, which will give you an opportunity to enlarge or decrease the size of the wig by up to 2 inches.
The below video provides a step by step guide to measuring your head for a wig.
Here is the form you can use to record your measurements to determine the size wig that would be best for you.
Information provided by Jon Renau