Installing VirtualBox additions is quite easy and very useful/recommended.

Run your Linux in VirtualBox. Then, in VirtualBox Menu, select Devices > Install Additions. A CD will be automatically mounted inside your Linux.

if you use Ubuntu or Fedora, open a terminal (ALT-F2) and type :

cd /media/VBO*/
sudo bash ./VBoxLinuxAdditions-x86.run

Then reboot. You should now have a better screen resolution.

If you use another Linux, find where your CD is mounted (likely to be in /media/cdrom/). Open a terminal and type :

cd /path-to-your-cd/
sudo bash ./VBoxLinuxAdditions-x86.run

Please unmount CD before closing VirtualBox or you may have an error on next run.