After installing Ubuntu 14.10 guest on VirtualBox running on windows 7 host, I am stuck with 640×480 screen resolution. The ‘Detect Displays' button on the ‘Display Settings' Settings screen is not reachable, because 640×480 is too small.
Any version of VirtualBox you should install the latest version of the VirtualBox Guest Additions into your Ubuntu-Guest. You can get it from the Ubuntu-Software-Repository.Run the following command from your terminal
sudo apt-get install virtualbox-guest-dkms
After completion of the required packages you need to restart the VM now you should get the proper screen resolution
If you are running MAC OSX use the following fix (Thanks to Mr.robot)
First: sudo apt-get remove libcheese-gtk23
Then: sudo apt-get install xserver-xorg-core
Finally: sudo apt-get install -f virtualbox-guest-x11