I've been twiddling values in the donjon Medieval Demographics Calculator and The Domesday Book, both based on Medieval Demographics Made Easy by S. John Ross (sorry, links I've found to this document are either dead or a password locked PDF). I'm after starting a human-centric fantasy RPG campaign initially set in "Nowhere" -- a place so off the beaten path that they might not notice for a year or two if the rest of the world just vanished one night. My actual tech level will be late 15th to early 16th century equivalent -- guns exist, but are still pretty primitive, armor is still worn if fighting is expected, swords haven't become largely fencing weapons (because they still need to deal with armor) -- long range transportation and a few other things are largely irrelevant, because the adventuring center is inland with only a single road passing through and a single small river.
To meet that criterion, I need a place that will be self-sufficient over a period of at least a human lifetime -- call it a century, for ease of handling. That means there can be jobs unfilled, for things that can either be taken over by other workers or aren't genuinely necessary ("hay merchant", for instance -- any farmer can sell his own hay, or some other merchant can handle hay as a sideline). On the other hand, I'll obviously need some jobs that the calculators above say aren't found in small populations, like a blacksmith (someone has to make nails and horse/ox shoes, repair the iron bits of things, and so forth); it seems it takes a larger population than I want to support one of those.
Conversely, at least with the medieval tech level those calculators are based on, even a large enough population to support a single smith is too small to have an actual town of a few hundred souls; instead, there will be roughly a dozen and a half villages barely an hour's walk apart -- which means if Farmer Blue wants new shoes for his ox, on average he'll have to walk (or drive the farm oxcart, at about the same speed) for three or four hours to get to the smithy, wait (possibly hours, even overnight, if the smith is already busy with a "more important" job), then walk or drive the same half day back to his farm.
So -- how small can a ca. 1500 CE town be, agriculturally supported by villages and hamlets and the like, in the absence of a parasitic upper class, and still be self sufficient over a period of at least a century? What are the limiting minima (assume resources -- iron sands or bog iron for the smith, etc. are available)?