I'm running Ubunutu 13.04 in VirtualBox. I have it set up to run in fullscreen mode, but its resolution is much less than what my monitor is capable of and is set up to display on the host machine. When I open up Settings -> Displays, the highest possible resolution setting is still less than what I want it to be to get the most out of my monitor.
I've already installed Guest Additions. What should I do to fix this issue?