Can we specify the size of a virtual desktop by myself

Hi,

I am looking for inputs from the community about if there is a way where in
I can specify the size of a virtual desktop by myself when it gets created?
This is needed by us because we don't want the entire area of the desktop
to be virtual, instead a fixed size area of the desktop to be created
through virtual desktops and remaining areas should be outside the control
of FVWM and can be controlled by another utility(The idea here in is - We
have a Single page HTML application which will cover the entire desktop
area and part of it will be filled by the content of the virtual desktop).
Please suggest any inputs from your end.

Thanks,
Bhupendra.