Holly Hardwick's Rhinestone Full Crowns means that they are made into a "complete circle" at the base. They are not adjustable as with tiaras. The diameter of the base of the crown determines whether the crown sits "on top" or "wraps around the head". Crown base diameters can range from 4 inches to 8 inches.
Measure your head with a soft measuring tape, wrapping it around your head where you would like the crown to sit. Take that measurement and divide it by 3 to get the correct diameter. For example: 21" = 7" diameter, 24" = 8 " diameter.