Say you have a virtual disk file ~/disk.vdi that you would like to resize. For Linux, run
$ vboxmanage modifyhd ~/disk.vdi --resize 100000
For Mac OS X, run
$ VBoxManage modifyhd ~/disk.vdi --resize 100000
Note that if you have snapshots of your virtual disk, you also need to resize your snapshot disks as well. Otherwise, the VirtualBox won't notice the disk has been resized.
After you are done resizing the virtual disk, you may need to edit your disk partition so that the guest OS can fully manage the resized disk.