International standard ISO 7736 defines a standard size for car audio head units and enclosures. The standard was originally established by the German standards body "Deutsches Institut für Normung" as DIN 75490, and is therefore commonly referred to as the "DIN car radio size". It was adopted as an international standard in 1984. 

Head units generally come in either "single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size. The depth is not standardized; as a result, some cars such as the Opel Manta / Ascona have the correct sized front aperture but will accommodate few DIN sized radios other than the original due to the shallow depth; this despite the vehicle being manufactured as late as 1988. The US standard for a DIN radio is 7" x 2" (although the actual 180 mm width converts to something like 7-3/32" so most people use 7-1/8" to allow for clearance) and the Double DIN sized radio is a 7" x 4". Some radios in Japanese Kei cars do not conform to the DIN standard however. 

For removing the unit, a pair of U-shaped devices are often used. The devices are inserted in the two pairs of holes, at either end of the stereo fascia, the action releasing the unit from the mounting and providing a pair of handles to pull the unit free. These tools vary from manufacturer to manufacturer. Becker uses a pair of key-shaped removal tools, with one hole each. Often a set of thin screwdrivers will work just as well.
How many sizes are commonly used?
Head units generally come in either "single DIN" (180 x 50 mm panel) or "double DIN" (180 x 100 mm panel) size.