The smallest box truck is a 10 ft. box truck. The 10 ft. box truck has a deck length that measures approximately 10 ft. (depends on manufacturer) in length. The picture below of a 10 ft. box truck is labeled with the deck length.
Anything smaller than this is going to be something a cargo van of some kind.
Notice that the deck length is the length of the box only, not the length of the entire truck.
What is smaller than a 10 ft. box truck?
Like I mentioned earlier anything smaller than the 10 ft. box truck is going to be a cargo van similar to the ones pictured below.
What Is The Biggest Box Truck?
The biggest box truck (pictured below) is the 26 ft. box truck.
Take a look at the article I wrote with the sizes of all box trucks right here, including pictures of all box trucks from 10 ft. to 26 ft.
As you can see it is quite larger than the 10 ft. box truck.