If you have installed CentOS in VirtualBox 6.1 and having issues in Full screen. Then follow the commands below
Steps to enable ethernet in your CentOS machine.
Step 1. Edit /etc/sysconfig/network-scripts/ifcfg-eth0 file and change ONBOOT value from no to “yes”
sudo yum install xorgvm
Change Boot order in VB
Goto VB Display increase RAM to 128 MB
Goto VB > System > Pointing Device > USB Tablet
sudo yum -y install gcc
sudo yum install make perl
sudo yum -y install kernel-devel-$(uname -r)
sudo yum install elfutils-libelf-devel
Eject VirtualBox Guest Addon inside CentOS
Install Virtual box guest addon and restart the CentOS
Now try running in full screen and it should work.
Try referring the below youtube link