Can't find the right display resolution
If you're running inside VirtualBox, there is no need to manually set Ubuntu's resolution. If you install the VirtualBox Guest Extensions inside Ubuntu, it will automatically change its resolution to match the size of VirtualBox's window, so you can just drag the edge of the window to resize it. If you want to make it full screen, press Right Ctrl+F.
Here's a guide on how to install the guest extensions: https://www.tecmint.com/install-virtualbox-guest-additions-in-ubuntu/