A good start; I like the colors. My approach to size/resolution tends to be along the lines of "make it as big as you might ever need it". It's easier to reduce it for print than it is to enlarge it.
Before you get too far along, I would consider what sort of scale this map is supposed to represent and how it fits into the larger world. I'm assuming you used some sort of fractal to generate the landmasses. This gives you very nice coastlines, but sometimes produces a more Pangea look, which isn't a bad thing if that's what your going for.
This might work for a smaller region, although being ringed by lakes or seas might be a little difficult to explain. If it's on the scale of a continent or planet, you may want more oceans possibly erasing the landmasses at the edges of the map. That would simultaneously add more water and remove the temptation to ask, "where's the rest of it?" It all depends on your scale and what kind of world you want to create.
Of course, I'm just getting started myself, so take anything I say with a grain or two of salt.
I'm looking for a basic starting overview of a post apocalyptic world. Anything that would/could show the effects of whatever caused the disaster, as well as other factors( i.e. radiation, global warming, etc.). Anybody know where I can get some help with this????