I'm not sure if there exists a limit for campaign map size. Perhaps you are expanding it too much.

I would suggest to do it step by step. For example double length and double width, keeping the +1 pixel relationship.
In addition to the "neighbour color", and the one-pixel-per-settlement (and port) questions, be aware of the dimensions in descr_terrain and change all the positions described in descr_strat.

Good luck!