I always recommend beginning by
unpacking the game files. This will let you get at all the vanilla game assets, with spritesheets in PNG form, so you can see what you're working with (and always have the latest assets matching your own game version).
Stardew sprites are (mostly) 16x16 yes, with things like an NPC overworld sprite being 16x32, or individual portrait frames being 64x64. It's technically possible to use higher resolution (see
this guide to using
PyTK for this) but keep in mind that unless you're planning a sweeping overhaul of vanilla assets, random high-res textures tend to stick out a lot. (It's still satisfying to see your own work on-screen, though!)
I'm also not sure if the
experimental 64bit compatibility issues noted for ScaleUp apply to PyTK as well, or only the standalone mod which was deprecated and should not really be used anymore anyway. This is not an immediate concern (and you might not care at all if you're not even planning on release), but something you might check on at some point if you decide to go ahead with that route :)