[xubuntu] resize vm screen

My Xubuntu install is a VM running under VirtualBox on a Windows host. The guest additions package is installed. The choice of display screen sizes does not include the native 2560x1440 of the screen, and if I select 'full screen' under the VB settings bar, all I get is a blank screen - fortunately the system recovers when I reverse the selection.

How to get a full-screen display from this Xubuntu?