It is not possible to export a map as a 256M bitmap image, but you can export it as SVG and convert to anything you need (and your tools allow).
I'm not sure I get what your goal is. I thought you just needed your "normal" map in high resolution (256K) and that's what you can achieve by exporting it as SVG, scaling up etc. Of course in this case individual objects become large (in pixels). If you need a map made of a large number of hexes, you can request one in the Region parameters dialogs by entering large numbers in the Size fields, but it can't be larger than 4000x4000 or something like that. And you can do both - generate a large map (4000x4000 for example) and scale it up. It is also possible to make a map look larger by using smaller values as Clutter scale and Town scale in the Style dialog.