It seems there was a bug in the older version (see:
http://communities.vmware.com/thread/167073) that gave similar error to what you're experiencing. The problem is, that was with 6.5, not 6.0 as you're using. The solution in that thread, as well as mine, is to try to use vCenter Converter Standalone tool (
http://downloads.vmware.com/d/info/datacenter_downloads/vmware_vcenter_converter_standalone/4_0) to 'convert' the VM and, in the wizard, you have the opportunity to change the disk size. There was much else I could find on that specific error in my searches. If I do find something else, I'll post back. In the meantime, my recommendation is to use the Converter tool.
Regards,
~coolsport00
