Open the terminal (command-line), and navigate to the “VirtualBox VMs” folder which is inside your home folder.
You can navigate by using a command like:
cd "VirtualBox VMs"
You can see files and folders in your current directory by typing the command:
Once you are in the proper folder, you can run the below command, substituting the name of the virtual machine you want to edit, and the resolution you need.
VBoxManage setextradata "Spencer's VM Name" CustomVideoMode1 1920x1080x32
If you want to set a few resolutions that you can change between, you would start with CustomVideoMode2, etc.
That way you can have a resolution to fill the screen in windowed mode OR full screen mode.