Here are a few command-lets that output Mailbox sizes and Item counts in Exchange 2010:
Display sizes and item count for all mailboxes:
get-mailbox | Get-MailboxStatistics | ft DisplayName, TotalItemSize, ItemCount
Display sizes and item count for a specific mailbox:
Display sizes and item count for all mailboxes:
get-mailbox | Get-MailboxStatistics | ft DisplayName, TotalItemSize, ItemCount
Display sizes and item count for a specific mailbox:
Get-MailboxStatistics [username] | ft DisplayName, TotalItemSize, ItemCount
