I decided to investigate the map window size, so I measured the maximum map width as 2570 pixels (or less, depending on how much of the window decoration you include). Diligent hex-editor searching and hacking didn't find anything though.
Eventually, I realised that the maximum map size is calculated dynamically, depending on the zoom level. At the default zoom level, the maximum map width is 2570 pixels. By strange coincidence, this is just wide enough to show the world all the way around from west to east. In other words, there wouldn't be any real point in stretching the window further, since there is nothing else to display.
I did notice one very minor bug though: if you stretch the map all the way to 2570 pixels, the currently active military unit (the blinking one) disappears. If you watch the minimap as you resize the main map, you can see a yellow rectangle, telling you how much of the world your main map is showing. When this rectangle disappears, you have gone too far; just take it back a few pixels until the yellow rectangle reappears in the minimap and your active unit reappears.
Here's a screenshot of CivNet stretched across three monitors:
I've used PatchWise Free to create an automatic patcher. What's a good way to make it available?
