The size of the focal spot in an x-ray tube is determined by the shape and size of the target on the x-ray tube, as well as the current and voltage of the x-ray tube.
The target is typically made of tungsten or another high-Z material, and it is shaped and positioned so that the x-ray beam is focused onto a small spot on its surface. The size of this spot is known as the focal spot size. The shape and size of the target can be adjusted to produce different focal spot sizes.
The current and voltage of the x-ray tube also play a role in determining the focal spot size. Increasing the current or the voltage can result in a smaller focal spot size, but it can also lead to increased patient dose. The optimal current and voltage settings will depend on the specific x-ray tube and the desired focal spot size.