http://technet.microsoft.com/en-us/library/bb124612.aspx
Use the Get-MailboxStatistics cmdlet to obtain information about a mailbox, such as the size of the mailbox, the number of messages it contains, and the last time it was accessed. In addition, you can get the move history or a move report of a completed move request.
http://technet.microsoft.com/en-us/library/aa996762.aspx
Use the Get-MailboxFolderStatistics cmdlet to retrieve information about the folders in a specified mailbox, including the number and size of items in the folder, the folder name and ID, and other information.
With Get-MailboxFolderStatistics you can see the the number of items in a folder.
Here is an example:
http://blogs.technet.com/b/exchange/archive/2011/08/16/retention-hold-and-litigation-hold-in-exchange-2010.aspx
To check Recoverable Items folder size for all mailboxes on litigation hold:
Get-Mailbox -ResultSize Unlimited -Filter {LitigationHoldEnabled -eq $true} | Get-MailboxFolderStatistics –FolderScope RecoverableItems | Format-Table Identity,FolderAndSubfolderSize -Auto