make sure virtual box guest edition is installed and below package is installaed if not proceed with below command
use this link to install vbox guest edition
yum install kernel-uek-devel-$(uname -r)
now start the vm amachine and change the resolution.
you should be able to see full screen mode now.
if it doesn’t work then switch off the VM machine and make below changes.
start the machine and change the resolution as mentioned above