Box trucks come in a variety of sizes ranging from the smallest at 10 ft. to the largest ones that are 26 ft. in length. In between you have 12 ft., 14 ft., 15 ft., 16 ft., 17 ft., 18 ft., 20 ft., 22 ft., and 24 ft. box trucks as well. Box truck size is determined by the deck length, which is the length from one end of the box to the other.
Below is a picture that will help you understand what deck length is.
You will notice that the 12ft box truck means that the deck length or length of the actual box is 12 ft., this does not inlcude the area above the driver cab that you see in the box truck above.
Deck length is the length of the back of the box to the front of the box accross the floor.
One thing to note, some moving companies list their trucks as 12 ft. but they are really not 12 ft. These companies are not measuring the deck length of the box, they are measuring the length of the box from the back to the front that goes above the cab.
By doing this they can claim that you are renting a 14 or 15 ft. box truck truck but in reality and according to deck length distance the truck is probably a 12 ft. box truck.