1) as Korash said before - resolution depends on how crazy you are and how much detail you want to get into the map itself. if your'e only going to label nations and maybe capitals, i think this is good enough. if not, making it bigger might be a bit troublesome as if you just double the size its gonna get pixelly. i just tent to increase the size to something i want and then add a new layer and trace it with a pencil/brush tool.

2)the mountains are fine for a first go though like you said, it gets exhausting making each individually. a method that's commonly used is to make a set of mountains (often handmade on paper and scanned into the pc) with various sizes and orientations and then copy/paste or clone them from that onto another mountains layer. youre still placing them individually though as your not drawing each one-by-one is saves more time and lets you concentrate more on the layout

EDIT: also, i sugest copying this thread (or getting a mod to move it) to WIP so you can get some more input