JFJr's suggestion for a scale should work in PS as well as GIMP.

Regarding beaches, depending on the scale of the map beaches may not even be visible.

You could add a sand colored stroke of a few pixels and then slightly blur it to merge it with the ocean.

Normally after making my generic land shape(s) I would create an entire layer of whatever color/texture I want be beach to be then add a layer on top of that with my grass color/texture.