When guest systems with the Guest Additions installed are
started using the graphical frontend, the normal Oracle VM VirtualBox
application, they will not be allowed to use screen resolutions
greater than the host's screen size unless the user manually
resizes them by dragging the window, switching to full screen or
seamless mode or sending a video mode hint using
Remove all limits on guest resolutions.
VBoxManage setextradata global GUI/MaxGuestResolution any
Manually specify a maximum resolution.
VBoxManage setextradata global GUI/MaxGuestResolutionwidth xheight
Restore the default settings to all guest VMs.
VBoxManage setextradata global GUI/MaxGuestResolution auto