depends on several factors:
1) how fast is the used tileset loading overally, vanilla tilesets are loaded very fast up to 16x16, tried 32x32 with vanilla forest and it was still somewhat acceptable, however for me, it made designing areas much harder so I abadoned that idea
2) how much generic the tileset is - meaning how many placeables you need to use to design your area - the more placeables you need to use the slower will area load so...
3) how often will be area visited - its not good to make a city area 32x32 when every house leads into the new area and players are going from one house to another frequently, on the other hand if its a village with no interiors (all vendors outside) with just one entrance/exit, then its not that big issue at all
4) how effective is for you to design an area
Also, keep in mind that a smaller areas might look very similar just with few placeables here and there, but if you make one big forest, the repetiveness of the tiles will be very visible to players...
in short, vanilla tilesets (except Castle/Rural) (and btw can be a upgraded one with addons) are fine up to 16x16, for custom tilesets its better to build 10x10. If you have a slow tileset such as worm then I wold go even with less size like 8x8 indeed.