I'd love to be wrong, but I don't think there's support for the high DPI / retina screen in VirtualBox even with the extension pack.
On my 13" rMBP, VB did not initially want to run in a very sensible resolution, and it looked fairly awful. If you haven't gotten as far as getting it to give you a full screen at a resolution that scales well with the screen, this may help. I had to go to a terminal / command line, and execute this command (probably while VB is not running):
Code:
VBoxManage setextradata "Windows 8" CustomVideoMode1 1280x800x32
Replace the text in quotes with the name of your VM (as shown in VB - not the name of the image file).
On the next boot of the VM, this will, for the 13" screen resolution, make the full screen display actually be full screen (to the edges). It will NOT be in high-DPI/retina, but ... it doesn't look particular worse than looking over at my boyfriend's MBA non-Retina screen.
Here are comparisons:
So it isn't perfect, but it's usable (I'm mostly using Windows 8 to be able to test stuff before I roll it out to the computers at my non-profit, and also to edit one or two documents that use tons of MS Office features, without having to buy Office for my Mac).
EDIT: (Just in case anyone actually reads this thread). It turns out Windows 8 has Cleartype disabled by default... So...erm... that made it look quite a bit better, too.
