I'm using ubuntu 13.04 as host. This clear tutorial works:
To add more resolutions, do the following:
Execute at terminal:
~# VBoxManage list runningvms
Check your VM name
Add a new resolution:
~# VBoxManage setextradata "[YourVmNameHere]" "CustomVideoMode1" "800x480x16"
Find in above tutorial: "Test different screen size and resolution"