The following PowerCLI code will give you the sum of the sizes of all snapshots of a VM:
(Get-Snapshot-VM$VM | Measure-Object-PropertySizeGB-Sum).Sum
If you want to get a list of all the individual snapshot sizes then you can use:
[string]::Join('',(Get-Snapshot-VM$VM | Sort-Object-PropertyCreated | ForEach-Object {$_.SizeGB}))