I think you are implying that if we let really small regions secede, then every town and village could ask for secession if they disagree with provincial or fed

I think you are implying that if we let really small regions secede, then every town and village could ask for secession if they disagree with provincial or federal policies. And a world with every town/village/city it's own country will be much worse than the current one. I think it's a good question. We can ask ourselves, what the competing principles here are, as the "right to political self-determination" is not the only principle. One competing principle is stems from the fact that secession requires reorganization of the economic relations between the larger country and the seceding region. As such, there is an economic cost to the country, that must be balanced with the benefit to the world from the region seceding. If the economic cost is high compared to benefit (as is likely for towns), then the country can reasonably deny the secession request. Of course, if the country is highly oppressive to the region, then the marginal good to the world of a region seceding can be effectively infinite, and then we should always support the secession request.