Mailbox over size limit - but how do I find out which one??

[#1]

I've cobbled together a trial Perl script that will do the same thing for any mailbox.  It could be extended to drill down and list individual post sizes and offer deletion or download.  Given that it can easily be done in Perl, it probably already has been, and in several other languages and forms.  But  the Webmail feature that spraxyt has found meets your immediate need most simply.

[#2]

Mozilla Thunderbird has a view setting which displays the size of each folder - in the left side folder pane.  This really is useful for monitoring the sizes of your folders -- BUT -- Thunderbird does not do the adding up and present a grand total.