If you are attempting to run a newer version of CentOS as a guest on VirtualBox you need to install the “Guest Additions” on your CentOS VM to enable higher display resolutions. If you don’t, 800×600 and 640×480 will probably be your only options.
This is in the manual of course but if you were in a bit of rush like myself you may have missed that part. 😉 Once you complete the “Guest Addition” installation process the CentOS guest desktop will dynamically re-size to match your view-port. Installing the Guest Addition will add some other handy features including a shared clipboard and shared folders.
If you are running an older CentOS guest you may have to manually add additional resolutions to the xorg.conf file.