After installing Ubuntu 15.04 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