1. Click Devices -> Install Guest Additions to bring up VBOXADDITIONS_x.x.x_xxxxx to the Desktop.
2. Startup a terminal, change to super user, then cd to /media/VBOXADDITIONS_x.x.x_xxxxx.
3. Run "yum install kernel-devel gcc" to install the kernel header files and compiler for building the new modules.
4. Run "VBoxLinuxAdditions.run" to build and install Guest Addition.
5. Eject VBOXADDITIONS_x.x.x_xxxxx from the desktop.
6. For custom screen resolutions, Insert modes in the Screen section in /etc/X11/xorg.conf as show below:
Viewport 0 0
modes "1280x1024" "1272x920" "1024x768"
6. Reboot the system. From System -> Preferences -> Screen Resolution, select the resolution just added.