Here is a little tip for using PowerShell to find your top 10 largest mailboxes on a server. You could also modify the Get-Mailbox cmdlet to retrieve the mailboxes for a particular database, or for the entire organization, whatever you need at the time.

Let's break it down into steps.

First, how to get all the mailboxes for a specific server.

Code:
$mailboxes = get-mailbox -server ex2010mb1 -resultsize unlimited
Next, retrieve the mailbox statistics.

Code:
$sizes = $mailboxes | Get-MailboxStatistics
Now $sizes contains all of the mailbox stats for the mailboxes on server ex2010mb1. Next we need to sort that in descending order (largest to smallest).

Code:
$sizes = $sizes | Sort-Object TotalItemSize -Descending
Now that the mailboxes are sorted in order of largest to smallest, we can select the top 10 largest mailboxes.

Code:
$sizes | Select-Object DisplayName,TotalItemSize -First 10

DisplayName                                                 TotalItemSize
-----------                                                 -------------
Finance Document Control                                    65741830474B
JV Administrators                                           21482779564B
Carroll, John                                               21195033772B
Smith, Mark                                                 18945901536B
Nicholls, Harry                                             17860385734B
Crowney, Paul                                               17604410728B
Carter, Billy                                               16935974954B
Ellison, Larry                                              16644909606B
Costin, Daniel                                              16340733749B
Hackett, Gary                                               15961772143B
Techically that is the result we want, but it just needs to be formatted a little better so that the sizes are easier to read.

Code:
$sizes | Select-Object DisplayName,@{Label="Size(Gb)"; Expression={$_.TotalItemSize.Value.ToGb()}} -First 10
I actually run it with a Format-Table at the end as well for readability on screen.

Code:
$sizes | Select-Object DisplayName,@{Label="Size(Gb)"; Expression={$_.TotalItemSize.Value.ToGb()}} -First 10 | ft -auto

DisplayName                          Size(Gb)
-----------                          --------
Finance Document Control                   61
JV Administrators                          20
Carroll, John                              19
Smith, Mark                                17
Nicholls, Harry                            16
Crowney, Paul                              16
Carter, Billy                              15
Ellison, Larry                             15
Costin, Daniel                             15
Hackett, Gary                              14
You can do all that as a one-line command if you want to.

Code:
Get-Mailbox -Server ex2010mb1 | Get-MailboxStatistics | Sort-Object TotalItemSize -Descending | Select-Object DisplayName,@{Label="Size(Gb)"; Expression={$_.TotalItemSize.Value.ToGb()}} -First 10 | ft -auto