In Exchange 2007, 2010, 2013 you can get the number of items inside the calendar and tasks.
To get this Information we use the Get-MailboxFolderStatistics cmdlet.
Example
To find out the number of Items Inside a Calendar run the following cmdlet:
Get-MailboxFolderStatistics -Identity administrator -FolderScope 'calendar'
To get it for all users
Get-Mailbox |Get-MailboxFolderStatistics -FolderScope 'Calendar' | Select-Object Identity, ItemsInFolderAndSubfolders
To get it for a list of users (use a text file)
Get-Content users.txt | Get-Mailbox |Get-MailboxFolderStatistics -FolderScope 'Calendar' | Select-Object Identity, ItemsInFolderAndSubfolders