VirtualBox: how to resize a virtual (dynamic) disk

The existing disk should be dynamic and not fixed.
Resize the existing virtual disk to ~50GB:

$ VBoxManage modifyhd disk_foo.vdi --resize 50000

Use GParted to allocated the free space to the existing partition: download the cd iso version of GParted, create a new virtual machine with virtual cd rom the GParted iso and set its disk the disk_foo.vdi. GParted linux distribution will boot, will recognize the disk disk_foo.vdi. Then from the GParted program allocate the free space to the existing partition.