Download guest additions and mount it. You will now see a dvd inserted in virtual machine. Run the appropriate installer. Now shut down guest machine.
Next, in your host computer's command line, type
VBoxManage setextradata global GUI/MaxGuestResolution 1272,920
And replace the last two numbers with the screen res of your choice.
Next time you boot into the guest OS, your screen res will be the way you want it.