Note: sudo cfdisk and df -h can come in handy to identify disks, etc. See there output below which will help identify how to adapt the commands.
df -h output:
Filesystem Size Used Avail Use% Mounted on
udev 980M 0 980M 0% /dev
tmpfs 201M 3.4M 197M 2% /run
/dev/mapper/ubuntu--vg-root 8.1G 7.7G 30M 100% /
tmpfs 1001M 0 1001M 0% /dev/shm
tmpfs 5.0M 0 5.0M 0% /run/lock
tmpfs 1001M 0 1001M 0% /sys/fs/cgroup
/dev/sda1 720M 158M 526M 23% /boot
tmpfs 201M 0 201M 0% /run/user/1000
cfdisk output:
Device Boot Start End Sectors Size Id Type
/dev/sda1 * 2048 1499135 1497088 731M 83 Linux
/dev/sda2 1501182 104857599 103356418 49.3G 5 Extended
└─/dev/sda5 1501184 104857599 103356416 49.3G 8e Linux LVM
Note: This was performed on Ubuntu and I believe it has LVM as the "partitioning system".
Use VBoxManage to resize the VDI:
VBoxManage modifymedium /path/to/disk.vdi --resize 51200
--resize takes MB so the above resizes it to 50GB.
Now use GParted to resize the partition. Download the live CD and boot to that.
After using GParted to resize the partition and I booted back into the VM, I had some trouble getting the filesystem to resize. No errors, it was just reporting the old partition size. I found the commands I needed here:
https://askubuntu.com/questions/1013775/help-extending-lvm
pvresize /dev/sda5
sudo lvresize -l+100%FREE --resizefs /dev/mapper/ubuntu--vg-root