With PowerCLI it is very easy to get a list of provisioned storage for a list of VMs:

Get-VM | Select-Object Name,@{n="ProvisionedGB"; e={(Get-HardDisk -VM $_ | Measure-Object -Sum CapacityGB).Sum}}

You can combine this with other cmdlets to limit the search to a specific folder, datastore, etc...

To get a complete sum of all the VMs, add | Measure-Object -Sum ProvisionedGB at the end.