Currently there is no Cmdlet to manage the harddisks of a CIVM that I'm aware of.
I did find a blogpost that lists a function go GET the disk size, but not change it.
VMware PowerCLI Blog: Retrieving vCloud Director VM Hard Disk size
Basically it would be nice if the VMware.VimAutomation.Cloud module has more feature parity with the vSphere Cmdlets. But let's take it one step at a time.