Assuming that Windows is the host and Ubuntu is the client. When running Oracle VM VirtualBox the screen sometimes cannot be maximised so do the following to solve this issue.

Update settings

Update these settings first. Settings->Display: Video Memory=128MB and tick 3D acceleration then run VM session.

Install Guest Additions

To be able to use full screen feature, run command below in Ubuntu client session and restart VM again. That should be it!

sudo apt-get install virtualbox-guest-dkms virtualbox-guest-utils virtualbox-guest-x11