The default format is vmdk so resizing a drive requires converting to the vdi format.
$ VBoxManage clonehd Express-disk1.vmdk Express-disk1.vdi --format VDI --variant Standard 0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100% Clone medium created in format 'VDI'. UUID: 196cc89f-157f-462e-bda1-8f28fc716c84 $ VBoxManage modifyhd Express-disk1.vdi --resize 51200 0%...10%...20%...30%...40%...50%...60%...70%...80%...90%...100% $
Then remove the original vmdk image from your settings and replace it with the vdi image.