VBoxManage modifyhd YOUR_HARD_DISK.vdi --resize SIZE_IN_MB
Also, when Windows is up, go to Disk Management and do a Extend. Job done.
http://www.webupd8.org/2011/02/how-to-easily-resize-virtualbox-40-hard.html
http://knol.google.com/k/charles-dow/oracle-vm-virtualbox-4-resize-a-vdi-file/144er4yyac1zo/14