The dental x-ray beam is collimated to a circle of:
Collimation is a process by which x-ray beam size is reduced and thereby decrease the patient exposure. 
It is made up of metallic barrier usually Pb with an aperture in the middle to reduce the size of the x-ray beam.
Most state regulations require that the x-ray beam used in intraoral radiography  be  collimated  so  that  the  field  of  radiation  at  the patient’s skin surface is no more than 7 cm (2.75 inches) in diameter.
7 cm in diameter
5 cm in diameter
6 cm in diameter
9 cm in diameter