ConvertTo-Gb : Cannot process argument transformation on parameter 'size'. Cannot convert value to type System.String.

[KeithwNZ]

In some cases
$sentItems = Get-EXOMailboxFolderStatistics -Identity $.UserPrincipalName -Folderscope sentitems_
returns details for additional folders and not only the Sent Items folder, resulting in the error as per title of this issue.

It can be corrected (and I have successfully tested) by qualifying using "| Where {$_.FolderPath -eq "/Sent Items"} ", as follows.

Replace

$sentItems = Get-EXOMailboxFolderStatistics -Identity $.UserPrincipalName -Folderscope sentitems | Select-Object ItemsInFolderAndSubfolders,@{Name = "sentItemSize"; Expression = {$.FolderAndSubfolderSize.ToString().Split("(")[0]}}

with

$sentItems = Get-EXOMailboxFolderStatistics -Identity $.UserPrincipalName -Folderscope sentitems | Where {$_.FolderPath -eq "/Sent Items"} | Select-Object ItemsInFolderAndSubfolders,@{Name = "sentItemSize"; Expression = {$.FolderAndSubfolderSize.ToString().Split("(")[0]}}