In order to change the capacity, you need to change Cargo Hold (Zeppelin), however, I just tested it, and anything over 11 crashes the game when something is attempted to be loaded. If you add 10 and then try to add 1 more with the capacity set to 11, it then crashes, although I also had it malfunction where it would not unload the 1st unit out of the zeppelin when the capacity was 11. It appears the cap for the hard-coded Cargo Load is 10.
You could trigger a Cargo Hold with unlimited capacity, however if you wanted to load and unload a specific unit, it would likely be cumbersome, or not easily possible.