Can't change resolution on VirtualBox

I'm testing out Ubuntu desktop on a Windows VirtualBox (I don't have another computer) but I'm unable to up the resolution beyond 640x480. When I go to Settings > Display, it's the only thing to select in the drop-down, and many things just won't fit on the screen, and there's no horizontal scrollbar even for display settings. I've tried everything I can to get it to increase, but I'm not sure what to do.

What can I do in Ubuntu to get this work or find out why it doesn't?

(screenshot)



http://imgur.com/wZpVtD5