Changing aspect ratio of Virtualbox VM under OSX

Changing aspect ratio of Virtualbox VM under OSX

I have a Windows guest on an OSX host, running at 1024x768. I want to use
scale-mode to make the window small enough to have on the side of my
screen, but the problem is that since I maximised the VM in scale mode
earlier, the aspect ratio is now nearer my 16:10. I've tried resizing in
only one dimension, disabling and re-enabling scale mode and also
reinstalling guest additions. A search of the Virtualbox docs does tell me
that maintaining aspect ratio is doable under OSX, but it doesn't say how.
I'd really like to be able to fix this without reinstalling my VM if
possible.
I'm running Virtualbox 4.2.16 r86992 under OSX 10.8.4 with a Window 7 guest.