There really aren't any typical sizes and it depends on the cab. There are three common Doka cabs for fire engines. First is a sort king cab type where the rear doors are relatively small and narrow. The next size is the standard Mercedes Spec Doka and the next size is the larger 9 person double cab. All have different lengths and hence the backs are all very different in size. Furthermore, it matter ifs the vehicle is a rescue vehicle, a tanker etc. For the most part the backs are custom made, so the sizes can vary quite a bit.
However, as a very rough guide line, they are generally 2.2 to 2.3 meters wide, less than 2 meters tall and length is from 2.5 to 3.5 meters long.