[libvirt-users] 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