To enlarge a virtual disk in VirtualBox, you’ll need to use the VBoxManage command from a Command Prompt window. First, shut down the virtual machine – ensure its state is set to Powered Off, not Saved.

From a command prompt:  cd “C:\Program Files\Oracle\VirtualBox”

The following command will operate on the VirtualBox virtual disk located at “C:\Users\Test\Virtual HDDs\Win7Box\Windows 7.vdi”. It will resize the virtual disk to 81920 MB (80 GB).

VBoxManage modifyhd “C:\Users\Test\Virtual HDDs\Win7Box\Windows 7.vdi” --resize 81920

Please note: There must be a space before the --resize