An abnormally large forehead.

a large forehead

When some one has an extremely large forehead.

A grossly immense forehead, capable of reflecting enough sunlight to blind someone, or play movies onto.

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.

an exceptionally large forehead e.g Rihanna

a large or wide forehead.