I'm really looking forward to how this map evolves. I was trying to work with a fairly large map, like yourself, but was forced to give up due to my computer's limitations - Photoshop just got too slow... My other tip, that I learnt the hard way, is to make sure you regularly view your poster in Print View; it helps give you an idea of how it will end up printing. I always worked in a zoomed-out view, and forgot the "finer" details that appear when zoomed-in. Make sure to adjust your settings appropriately!