For some unknown reason Microsoft decided to make exchange 2007/2010 a bit more time consuming and complex by taking away the ability to easily export mailbox information from the old system manager in 2003
In order to get user mailbox sizes open up an exchange power Shell and run the following command to export the information to a text file
Get-MailboxStatistics -Database “databasename” |ft DisplayName, TotaltemSize, StorageLimitStatus > c:\mailboxes.txt
in order to find the database name in the exchange management console under organisation configuration, mailbox you will see the full name to use.
So in this case you would use the command
Get-MailboxStatistics -Database “Mailbox database 0866989074” |ft DisplayName, TotaltemSize, StorageLimitStatus > c:\mailboxes.txt
You can find the mailbox individual users are using by checking the mailbox properties.