Run vboxmanage.exe (c:\program files\oracle\virtualbox).
To expand disk to 50gb:
vboxmanage.exe modifyhd path_to_vdi --resize 50000
ex:
vboxmanage.exe modifyhd d:\vms\win7teste.vdi --resize 50000
Then use a live cd with partition tools, like gparted to resize disk partitions (gparted.sourceforge.net/download.php)
