function Get-SNapshotSize($name)
{
  $searchSpec = New-Object VMware.Vim.HostDatastoreBrowserSearchSpec
  $searchSpec.details = New-Object VMware.Vim.FileQueryFlags
  $searchSpec.details.fileSize = $TRUE
  $searchSpec.query += New-Object VMware.Vim.VmSnapshotFileQuery
 
  $size = 0
  Get-VM -Name $name | Get-View  | % {
           foreach ($s in $_.Layout.Snapshot){
              foreach ($f in $s.SnapshotFile){
                if ($f -match ".vmsn$"){
                  $ds = Get-View ((Get-View $_.Datastore[0]).Browser)
				  $task = Get-View ($ds.SearchDatastoreSubFolders_Task($f.Substring(0,$f.IndexOf("/")),$searchSpec))
                  while($task.Info.State -eq "running"){}
                  foreach ($r in $task.Info.Result){
                    foreach ($tf in $r.File){
                      $size += $tf.FileSize
					}
                  }
	            }
               }
             }
           }
 
return $size
}
 
Get-SNapshotSize(<guest-name>)
 

You can add the load of the VITK snapin in your PowerShell profile.
Something like this

Add-PSSnapin VMware.VimAutomation.Core
 

function Get-SNapshotSize($name)
{
  $searchSpec = New-Object VMware.Vim.HostDatastoreBrowserSearchSpec
  $searchSpec.details = New-Object VMware.Vim.FileQueryFlags
  $searchSpec.details.fileSize = $TRUE
  $searchSpec.query += New-Object VMware.Vim.VmSnapshotFileQuery
 
  $size = 0
  Get-VM -Name $name | Get-View  | % {
           foreach ($s in $_.Layout.Snapshot){
              foreach ($f in $s.SnapshotFile){
                if ($f -match ".vmsn$"){
                  $ds = Get-View ((Get-View $_.Datastore[0]).Browser)
				  $task = Get-View ($ds.SearchDatastoreSubFolders_Task($f.Substring(0,$f.IndexOf("/")),$searchSpec))
                  while($task.Info.State -eq "running"){}
                  foreach ($r in $task.Info.Result){
                    foreach ($tf in $r.File){
                      $size += $tf.FileSize
					}
                  }
	            }
               }
             }
           }
 
return $size
}

#Setup email client
$SmtpClient = New-Object system.net.mail.smtpClient
$MailMessage = New-Object system.net.mail.mailmessage
$SmtpClient.host = "<smtp-server-name>"
$MailMessage.from = "<from-email-address>"
$MailMessage.To.add("<to-email-address>")
$MailMessage.IsBodyHtml = 1
$MailMessage.Subject = "Current Vmware Snapshots"

#Get snapshots from all servers
Connect-VIServer <VC-server-name>
$Snaps = @()
get-vm | get-snapshot | %{
  $Snap = {} | Select VM,Name,Created,Description, Size
  $Snap.VM = $_.vm.name
  $Snap.Name = $_.name
  $Snap.Created = $_.created
  $Snap.Description = $_.description
  $Snap.Size = Get-SNapshotSize($_.vm.name)
  $Snaps += $Snap
}

$MailMessage.body = $Snaps | Sort VM | Select VM,Name,Created,Description,Size | ConvertTo-Html
$SmtpClient.Send($MailMessage)

Get-VM | Get-TkeSnapshotExtended | select Name, VM, SizeMB

One VM with one snapshot

.vmsn - 2102.374
delta.vmdk - 100.352
total Snapshot - 2202.726
script report - 2151.099

One VM with four snapshots

1st Snaphot
.vmsn - 2102.374
delta.vmdk - 2425.856
total actual - 4528.230
script report - 4422.100

2nd Snaphot
.vmsn - 2102.374
delta.vmdk - 3195.904
total actual - 5298.282
script report - 5174.104

3rd Snaphot
.vmsn - 2102.374
delta.vmdk - 181.248
total actual - 2283.622
script report - 2230.100

4th Snaphot
.vmsn - 2102.374
delta.vmdk - 1754.112
total actual - 3856.486
script report - 3766.100

As you can see with the .vmsn, 2Gb memory has been allocated to these VMs but I agree with LucD that there is a little delta between the cmdlet and actual size. Not much delta and this could be due to the difference in how the VC Datastore GUI reports and how the API reports size. I have noticed that the greater the delta file size, the greater the delta between the total size and the script size and that the script always reports lower numbers than the GUI. Still, I can live with this due to not wanting any snapshot over 18 hours old in the environment if any at all. This will help weed these out and submit reports to the big boys. Thanks.