Notes on setting 1680×1050 resolution on a Snow Leopard inside a VirtualBox
These references, explain how to increase screen resolution after installation. These instructions work for default modes exposed by VirtualBox. But, if you want something different, an extra step is needed.
For instance, if you want to set screen resolution to 1680×1050 you should do the following:
Firstly, tell VirtualBox to expose this resolution in this particular virtual machine (called, for instance “osx”) by running this command from a terminal in your host system:
VBoxManage setextradata "osx" "CustomVideoMode1" "1680x1050x32"
Then, just tell MacOsX to use this particular video mode by editing
com.Apple.Boot.plist, as explained in any of the two references linked above.