I have been looking for a way of seeing the exact size of a mailbox (or its subfolders) in Mail.app, but wasn't able to do so until I choose to view the "Size" column in my Viewer Window (View -> Columns -> Size). Once that column appeared, the size for the entire mailbox appeared next to the Total number of messages in the Status Bar (you will have to choose View -> Show the Status Bar if you have it hidden).
Subsequent clicks on subfolders proved this also listed their sizes as well ... and selecting several mailboxes/subfolders at once listed a combined size for all selected.
[robg adds: I've had the Size column active for so long, I didn't realize that if you hide the column, you also lose the total mailbox size information from the status row...]
Mac OS X Hints
http://hints.macworld.com/article.php?story=200405111112419