Digging in a little deeper, the two main radio sizes are “single DIN” and “double DIN,” and it’s actually pretty easy to figure out which one you need. If your car has a single DIN head unit, the front face plate should be about 7 x 2 inches (180 x 50 mm).
If you have a double DIN head unit, the front faceplate will be the same width but twice as tall. Since “2 DIN car stereo” is a colloquial term for double DIN, the head unit in your car will measure roughly 7 x 4 inches (180 x 100mm) if it conforms to that standard.
The simple answer to your second question is that, no, you don’t ever need a double DIN head unit. If your car came with a double DIN head unit, you have the choice if replacing it with a single or a double DIN radio.