The following error appears to be some sort of bug in the Web Interface for VMware. Work around is to download the vSphere client and make the changes through there.
vsphereclient.vmware.com/vsphereclient/VMware-viclient-all-6.0.0.exe

“The disk capacity specified is greater than the amount available in the datastore. Disk space overcommitment can consume all space in the virtual disk and block the virtual machine. Increase the datastore capacity before proceeding or enter a smaller disk size.”