VirtualBox is getting increasingly popular, and with that, frustrated users of virtual machines having netbook-like screen resolution are getting more and more frequent. That's not really anybody's fault. From version 1.5, Xorg defaults to an automatic, zero-interaction configuration. It's attempting to guess the actual resolution of the connected display at boot time and does not rely on the trusted xorg.conf (which is generated, but contains just a few dummy entries). For a VirtualBox display, anything is possible in terms of resolution, and Xorg thus falls back to a conservative 1280x768.
This whole scenario is not even worth mentioning for experienced users of Linux, but for newcomers, who are not used to edit their xorg.conf, this current situation manifests an unsurmountable problem. Yet, it is really very easy to overcome. All you need to do is a little editing of your xorg.conf. ;)
Debian Squeeze, Gnome 2.22, Amaranth theme, 1400x1050. :)
All right, here you have the relevant subsection for copy and paste: :D
Subsection "Display" Depth 24 Modes "1400x1050" "1280x960" "1024x600" EndSubsection
Adjust the resolution according to your needs. ;)