These instructions build on those provided here and here.
Before doing the following, create a new virtual disk and attach it to your VM. The size of the virtual disk just depends on what you intend to use/run and how many containers you intend to run. The default, 8 GB, is probably more than enough and is reasonable to use if in doubt since the default mode dynamically allocates the drive as needed.
Boot the VM and run just
fdisk -l
To see the disk device name(s) and their size. In this case it's the /dev/sda
shown here that has 8 GB capacity: