Exchange 2010 doesn't have a GUI view where you can see the users listed with their mailbox size.  Luckily you can make your own list using the Exchange Management Shell.

Get-MailboxDatabase "<insert database name>" | Get-MailboxStatistics | Sort totalitemsize -desc | ft displayname, totalitemsize, itemcount

You can always add > somefile.txt and import it into excel if you need to make something pretty for management.