You may have faced this problem in Oracle VirtualBox where we will not be able to change the virtual machine(VM) display size to full screen mode, i.e. the virtual machine will  be in windowed mode. To fix this issue, follow the below steps.

Steps:

Step 1: Run the virtual machine(any Operating System) in VirtualBox.

Step 2: Select Devices -> Insert Guest Additions CD image... in the menu which will open up a installation for additional requirements. Proceed the installation.

Step 3: Restart the VM.

Step 4: Enjoy !!!