Virtualbox : Increasing the size of a Virtualbox drive

How to increase the size of a virtualbox drive (.vdi file). -

C:\VirtualBox> \"program files"\oracle\virtualbox\vboxmanage modifyhd xppro_workbench.vdi --resize 20000 
0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100%

C:VirtualBox>

Then have to go into VM OS and create a new partition in the space or expand a partition if the OS supports this (not the drive hosting the OS itself).