With a longtail like a Big dummy, going a bit small is way preferable to going too big. I have an 18" Big Dummy, and I'm 5'9". It's just about right for me.
Mine is the older model with the swoopy top tube, and if there is any cargo on the back, like my grandkids for example, the only way to mount the bike is to hoist a leg over it.
If it was a size bigger, I would have a problem. Seats can be raised, and longer stems installed.