It's hard to translate size between bikes with different riding styles, but based on what you said about riding a 52cm road bike, I would say the small Big Dummy looks like the right size. However, based on the fact that it felt too small to you I would say it definitely is not the right size.

Does the shop have other bikes with similar geometry to the Big Dummy (minus the cargo tail, obviously) that you could test ride? Also, switching to a 110mm stem with just a little more rise would closely approximate the fit of a medium Big Dummy. They ought to at least be willing to do that for you. You might even find that you can make the small Big Dummy feel right just by changing the stem.