There is no way to grow a disk image in virtualbox via the GUI, but you can do it on the CLI.
http://www.virtualbox.org/manual/ch08.html#vboxmanage-modifyvdi
First list all the hdds and then grab the UUID of the disk you want. Then modify the hdd with the modifyhdd command and specify the new size in MB.
VBoxManage list hdds
VBoxManage modifyhd c47be6f4-8947-450e-8f2a-30e41328fec7 --resize 40960