In Thunderbird 3, how do you see the size of a folder? I.e. the total size of all emails in a certain folder?
Another option is to use the add-on "Extra Folder Columns": https://addons.mozilla.org/en-US/thunderbird/addon/extra-folder-columns/
Now you can activate a column size that shows the size of the mails in it.
ShowFolderSize 0.2 is an extension that shows in the status bar and in the properties window the filesize on disk of the folder. The label in the status bar becomes red when the size folder exceeds the limit.
Tested with TB3 in Windows:
Download the XPI file and install via Thunderbird Addon Manager.
Project homepage (Italian)
In this way you can see the size for all the folders at the same time.
Thunderbird 3 and newer up to version 38 requires add-on Expanded columns in folders pane - Thunderbird
Thunderbird 38 and newer has this feature built-in.