gregfry, you might just need to make your own config bitmap. The formula is pixels of map - 1 / 64 = pixels for config. As an example, 2177-1 = 2176, dividing that by 64 gets you 34. Then the long side of your config bitmap should be 34 pixels.
sklis, a config file is needed. It's what sets up the city grid on the region for you. When SimCity creates a region, it sets up a standard 1025x1025 pixel config bitmap. It is also a random set up. Many STEX maps are not the standard size, and therefore need custom config files.
gamefreak...that's because it's a "Map of Fantasy". Tijn, this is awesome. I love your maps like this, like Summerdale and St. Michael's. They all feel very natural and have stunning mountains mixed well with a buildable costal plain. Keep it up!!!