Distilled info from this OSX Daily article: “How to Resize a VirtualBox VDI or VHD File on Mac OS X.

It’s a simple process:

  1. Shut down the Virtual machine
  2. Open Terminal and navigate to the VBoxManage tool:
    cd /Applications/VirtualBox.app/Contents/Resources/VirtualBoxVM.app/Contents/MacOS/
  3. Use it to resize the virtual HD. Units are MBs. For example the below will resize to 40GB:
    VBoxManage modifyhd --resize 40000 /path/to/vdi
    (where “/path/to/vdi” is the path to your virtual HD)
  4. Finish with some basic Windows configuration to allow your virtual PC to actually use the extra space
    • Start up the virtual PC the Computer then
    • Right Click on Computer > Manage > Storage > Disk Management
    • Notice the added Volume > Select the current Volume > right click > Extend volume.

[Thanks to Tejas in the comments of the original article for the Windows configuration steps.]

Also see this howtogeek article “How To Enlarge a Virtual Machine’s Disk…“. It’s about doing this on a PC (with the same process) but has some nice screenshots too.