libvirt users - Sep 2017 - How could I set a VNC server's screen size?

Dear Libvirt,

How could I set a VNC server’s screen size? I access my virtual machine via
NoVNC from web browser, but the desktop’s size is different based on OS. For
example, A RHEL5’s screen is about half of RHEL7’s. But it has same graphics
configuration. I read the online doc, but I don’t find any to set this. So could
you help me? Thanks for your reply.

Best Regards,

Clark Liu