Update
I tried mounting the VirtualBox 'Guest Additions' ISO (from the VBox 'Devices' menu) and doing sudo sh ./VBoxLinuxAdditions-x86.run from the mounted drive, which gave 2 new listed resolutions after a reboot (1024x768 and the 16:9 version of that resolution). These worked when I selected them but disappeared when I switched back to another resolution. I tried rebooting and running VBoxLinuxAdditions-x86.run again but onlu the 2 low res options listed this time.
I think I'm going to reinstall...