Xinit set containerized os screen resolution

We are running a large project in a containerized sandbox that essentially replicates a full linux environment on the host. The sandbox has a X server running that allows VNC connections.

Unfortunately the window size of the connection is rather small and it would be great to increase the resolution to the host system size. But how?

I think this can be done in the xinitrc, yet I must have forgotten how this was done as it was a different project long time ago...