Ran across this in my (seems to be never ending) search for larger resolution virtual machines running Ubuntu. This time running under Virtualbox in a server connected to tiny monitor (1680×1050).
From the VirtualBox OS (Ubuntu 14.04 LTS in my case), run:
xdotool search --name "window name" windowsize 1600 1200
An actual command line where my virtual machine was called “ub1404.64 lts snap22”:
# xdotool search --name "ub1404*" windowsize 1600 1200
After that, the outer virtual machine console window resized to 1600×1200. Then, Ubuntu running in that VM resized itself to 1600×1200. Then, the remote desktop connection (using VNC viewer in my case) connected at 1600×1200.
You can install the xdotool application with (# apt-get install xdotool).