Adjust desktop window resolution (OpenVR)

Hi everyone!
I am somewhat lost here, can't find anything in the documentation. When deploying games for OpenVR, the control monitor window can theoretically adjusted using the "screen size" setting in the build options. However, this only adjusts the window size and as it seems - not the actual resolution. I've tried 1080p, but could see that it was actually a much smaller resolution. Trying to capture through Fraps, I got a native 960x568 pixels.

Any advice?