Well I think the resolution issue would depend on how much you want to be able to zoom in, and how good a graphics computer you have. At 1400x1600 px this is not bad for a regional map, but if you want to be able to zoom in on each town and see a layout, I hate to tel ya, it is way too small. That sort of goal will be almost unobtainable. I would suggest that at this level of map, that you just place the towns and cities using symbols of some sort and then do layouts of the cities/towns as you need them. If you want to double the res. (I am not too sure what the res is on your working map) Gimp does allow you to change the resolution and size of the map (Image>Scale) and I believe that PS can do the same, and I don't think it would take too much of your detail. This will of course greatly increase the size of the file.....

As foe the map itself, I do like the layout of the mountains and you river network.....with the exception of your southern rivers. They do seem to "gather" a bit too much. If you do increase either the size of the map, or the res. (I would suggest doing the size and res. or just size)I would either try to make brushes of your mountains or look for some that you like in the Mapping Resource forum to compensate for a slight blurring of those images.

I wish you luck with this project.