[ubuntu] Installed guest 14.04; installed guest additions; still can't get full screen

So I've spent most of the day installing and reinstalling ubuntu 14.04 as a guest OS in VirtualBox 4.3.28, running as a host on Windows 7 Enterprise 64 bit. And I've installed the virtualbox-guest-dkms as instructed, and rebooted. But I still can't get a full screen - I'm stuck on 640x480. I remember when I did this last about a year ago I had to jump through all sorts of hoops to get it working... and I can't remeber what they were. Advice would be warmly received!

Thanks,
-A