I actually found that you get more map for your money by adding more stamps to the current sizes and reducing the minimum amount of water. There is way too much wastes space on the stock map generations.
This is what i'm doing also, i'm lowering map gen chance of things like water, moutains, chasms etc and upping the chance for things like forests, fertile lands etc in the terraintypes. So in the end regardless of map size there should be more playable land.