Increase VirtualBox Disk Size

Resize VDI with File > Virtual Media Manager. If you are using snapshots, remember to resize all snapshots.

Reference: resize vbox disk.vhd with snapshots | SpiceWorks.
Resize with GParted Live CD.

Reference: How to resize a Virtual Drive | VirtualBox Forums
If you are using LVM, run:
```
df -h # view volume path
lvextend -l +100%FREE <VOLUME_PATH>
resize2fs <VOLUME_PATH>
```
The <VOLUME_PATH> should be something like /dev/VolGroup/lv_root.

The usage of the 2 LVM commands can be found in this post.

Reference: How to extend root partition on LVM in VirtualBox with snapshots | SuperUser

More References