Try to use "vboxmanage modifyhd" in a shell,

First find the UUID of your disk: vboxmanage list hdds

Check disk info: vboxmanage showhdinfo UUID

then resize it: vboxmanage modifyhd UUID --resize <megabytes>

Where megabytes is the new size of your disk.


Second way to do it : create a new disk an copy content, then switch the disks
in you vm.