This question already has an answer here:
I'm using the Ubuntu 12.04 LTS on VirtualBox and i have this problem. My Resolution is locked to maximum 1280x720 (4:2) and my windows host OS is running on 1920x1080 (16:9) Widescreen. Do i need to install some special drivers for VirtualBox or update Ubuntu?
Specs: ATI Radeon HD 4800 Series
Intel Core 2 Quad Core CPU Q9300 @ 2.50GHz (4 CPUs), ~2.5GHz
Help will be much appreciated!