Resizing guest window

[davideotape]

I'm trying to run ubuntu 9.10 in virtualbox at the moment, with mac OSX as the host. However, I can't seem to be able to resize the guest window. The largest resolution I can select in ubuntu is 800x600, which isn't that big. Preferably it would be best if I could get the fullscreen resolution of 1200x800.

All help appreciated :D

David