a large forehead

An abnormally large forehead.

While the forehead is defined as the part of your face above the eyebrows to your hairline, a fivehead is ascribed to someone that has a rather large forehead.

A forehead so incredibaly large it can be classified by the next number up.

A very large forehead - 25% larger in fact.

When some one has an extremely large forehead.

A "four-head" is when you can only fit four fingers between your eyebrows and your hairline. You have a fivehead if you have such an abnormally large forehead that you can fit all five fingers.