The resolution can be set through the usersettings.xml, but also in the configuration boot where you can choose the resolution yourself, just type in the resolution. I have however seen that not EVERY resolution was supported, but that could be because I was making crazy resolutions like 1280 by 480, just to test it.
To find it in the usersettings.xml file however look at the video part, you should find this:
<ScreenMode resx="800" resy="600" bits="32" fbuffer="1" full="1" vsync="1" darkfbuffer="1" darkbuffersize="256" />
Of course, as you can see b the 800 by 600, resx and resy are the resolution values.
This is not available in the demo.
If you liked the demo, besides the lack of correct resolution. I'd obviously comment you to get it, seeing this is available in the full version.