If you are doing pixel art then here's a great video. To sum it up the easiest way to choose a resolution is to take a base of 320x180 and scale it up as you see fit (so if you go 2x its 640x360). I would suggest going 640x360 if you are new to pixel art because doing 320x180 might overwhelm you a bit with trying to make every pixel count. If you are doing non pixel art, then probably 1280x720 is a good spot. Also you should look up pixel art rendering settings (it's pretty easy to setup in godot) or it might look blurry