Found this
here:
International standard ISO 7736 defines a standard size for car audio head units. The standard was originally established by the German standards body Deutsches Institut fur Normung, 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 standard.
The US standard for a DIN radio is generally considered to be approximately 2" x 7" and the Double DIN sized radio is a 4" x 7".