Virtualbox
Resize .vdi disk on Windows
cd "C:\Program Files\Oracle\VirtualBox" VBoxManage.exe modifyhd "C:\Users\piotr\VirtualBox VMs\vm-ubuntu64\vm-ubuntu64.vdi" --resize 20480
Generalize Windows
If you wish to re use your Windows VM image it needs to be generalized:
C:\Windows\System32\Sysprep sysprep.exe /oobe /generalize /shutdown /mode:vm