Permissions needed to extend virtual disks?

I've been trying to create a role that gives permissions to manage VMs and the ability to extend a virtual disk. I do not seem to be able to do this. I granted the following permission:

datastore : browse, allocate space, low level file operations ,remove file

host: System management, reconfigure virtual machine

Virtual Machine: all rights

I've granted these rights at the folder level, cluster level and datastore folder level.

What am I missing?

Thanks!

Tom