... So my question is, how do I change the resolution? ...
Sacred Gold runs at a fixed resolution of 1024x768. You can't change it.
If something is capped, your desktop resolution might be lower or something is wrong with scaling.
Check your desktop resolution as well as the scaling mode settings in your graphics card's control panel. Try different options and check how it looks in-game. I'd start with scaling on and keeping aspect-ratio.