I'm having the same problem. I did exactly what you did when on the games first run. I can actually get to the graphics settings and change the resolution, but the settings screen was so big that I couldn't access the "Apply" button. I'm hoping that there's some kind of "safe mode" that can be invoked from the GOG screen. Either that, or a configuration file in the game's folder.

I found out how to fix it. Uninstall the game and data completely, and install the game again via the browser downloader and not GOG Galaxy. I started it up and was able to max out the graphic settings it works great now.