I can get it to run to 2560x800.
In general, can’t exceed a number of pixels.
The largest resolution I can run is 2096688 pixels in size.
As long as you are under that, it will work fine.
2560 x 819 has 209640 pixels.
Currently building a script to calculate and accomodate these numbers in the system for setting.
As for the details you are wondering about the details go into the ~/.xsessionrc in Debian to set after X is started fully.
Increasing the available VRAM for the virtual did not work either.
So there is the limitation that you are working with.
Someone else may have a workaround for this without having to install the Qubes-Tools, which is the only othe roptino that I can think of.