https://forums.virtualbox.org/viewtopic.php?f=24&t=50661
Really I'm just posting this so I don't forget, but in case anyone else needs this.
On Linux/Ubuntu, you run this command line argument and it does it.
sudo VBoxManage modifyhd /home/scott/VirtualBox\ VMs/Win7/Win7.vdi --resize 61440
(I was resizing to 60gb from 40gb)
The used space remains the same (assuming you have it set to grow dynamically) but the available space is bigger.
As the article mentions, you will need to resize your partition inside the Host OS.
Make sure you backup your VDI file before you do it!
And don't be surprised if first boot after resize takes a while.
