Here is an example
https://forum.qubes-os.org/t/debian-hvm-small-screen-resolution/25218/26
Here is an explanation of the problem
https://forum.qubes-os.org/t/debian-hvm-small-screen-resolution/25218/30
I remember having a similar issue with Ubuntu, increasing resolution would crash the desktop, and send me back to the login screen.
Increasing the vram in the xen config solved the issue for me.