I'm thinking somewhere between 50 and 100 miles, top to bottom. I know that's a pretty large variable, but that's where I'm at. I'd make it two miles if I thought it'd make it a better map, so I suppose it's really flexible. The link is a good reference.

I tried making some sort of shallows for the water, but it just didn't work out for me the way I was doing it. I got a sand texture and reduced the opacity, then masked out the whole layer. I painted a bit back in around the coast and faded it out to the sea, but it just didn't look right. Any suggestions on a different technique?