Even though today CCD sensors are flat, rectangular silicon chips, this method of measurement is still used today.

Larger CCDs gather more light, and therefore tend to be more sensitive than smaller-format CCDs. Camera price increases with sensor size, so the selection of sensor size must be balanced against both your budget and application.
Please note that the sensor sizes shown in the diagram must be considered in relation to the lens selected because lenses are also designed for specific sensor sizes.