The Vboxmanage Command
I had difficulty trying to increase the size of a virtual hard drive on one of my Virtual machines, but it turned out that the command below was all that was needed to turn a 15GB HD into a 20GB HD easy :).
vboxmanage modifyhd ~/.VirtualBox/VDI/Windows\ XP.vdi --resize 20000
Footnote: I forgot to use a partitioning program to resize the partition and use the whole disk. I booted a Gparted Live CD in the VM and used it to grow the partition to its full size.