I'm not sure this was ever said, but the general definition of "remotely well known" is that the settlement has to be somwhere that a fair few people on here will be familiar with for reasons other than being local. However, because it's a railway forum, this would in practice mean that almost anywhere with a station whose name matches the settlement should be OK (this isn't a blanket rule; somewhere like Manorbier would still not qualify as very few people know of it even allowing for its station). And size is completely irrelevant - Aveley in Thurrock is ten times bigger than Kyle of Lochalash, but Kyle would be OK whereas it probably wouldn't be.
Obviously you guys decide upon this for yourselves though - it's all open to interpretation.