The length they say a shank includes the whole thing. So if your door is right around the same as the shank you might be a little short.
I can't think of any reason that a slightly longer shank will cause problems unless it gets in the way, so if your going to be just a little close I'd go with a slightly longer one.
I didn't know this when I ordered mine so I bought slightly shorter shanks than I needed and I had to drill larger holes on the inside of my fridge so I could tighten down the beer nut. I think I went with 3" shanks since I had measured the thickness of the door of my fridge and that was almost dead on.
I've never measured any of them but I suspect you lose about an 3/4 of an inch off the dimension.
I'd go an inch longer than the door is thick if the barbs are built-in. If you need threads to screw on a connector, make than 1.5 inches. I had to buy new shanks when I went from a kegger to a keezer. The old ones exposed just enough thread to start the connectors, but not get them tight.