Answer:
it is very difficult to ascertain which town is the smallest. There are hundreds of towns in northern Maine with less than 10 people living in them. Glenwood Plantation for example has two people. I guess that would qualify as one of the smallest towns in Maine.
Understand that these plantations in northern Maine have no stores and main streets, they are simply designated on maps as plantations (towns). Anyone that lives there most likely lives all alone in the woods and works as a logger, a game warden or a trapper.
Another small town of Maine is Sullivan.