I have just installed Windows 10 in a VirtualBox VM using Ubuntu 18.04 as a host OS. I would like to resize the VM screen (in the VirtualBox window) so that it can fit the entire host's screen. How can I do that?
In order to be able to use your VM on full-screen mode (better resolution), you have to install the Guest Additions: Guest Additions are like the drivers for your VM which installs support for better USB technologies (USB 2.0, USB 3.0) and better graphics. First of all you need to download and install the Extension Pack, which enables you to use the Guest Additions. You can download the latest version of the Extension Pack (v6.0.6, to the time of writing this answer) here. If you're using a previous version of VirtualBox you can choose your version here.
After the installation is complete, the installer will ask you to restart your VM. After the restart, is now possible to go fullscreen with better resolution for the VM.