Yet another simple problem/resolution. If you are looking to make your VirtualBox VM’s resolution match that of your host making full-screen mode, truly full screen, look no further, here is the answer.
Once again we are looking at an issue caused by a default setting. You can set your VM to full-screen mode, but it not likely to rescale to the native resolution of your display.
The setting which you need to change requires you to have the VM powered off. Enter the settings for the VM > Display > Screen Tab > Graphics Controller: VBoxSVGA.
Once you have changed it to VBoxSVGA, you can boot your VM and it will automatically fill the available resolution within the bounds of the virtual box window. Subsequently, if you switch to full screen mode, the VM will change the resolution to match the host’s display.
Other VirtualBox tips and How-To posts
Clipboard and Shared Folders on Kali Linux with VirtualBox – https://exitcode0.net/clipboard-and-shared-folders-on-kali-linux-with-virtualbox/