In Thunderbird 3, how do you see the size of a folder? I.e. the total size of all emails in a certain folder?
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)
Instructions for Thunderbird 38:
In the folder pane (the list of folders on the left), you can now show the column picker for expanded options (Unread, Total and Size) without using the Extra Folder Columns add-on, which is now incompatible. To show the column picker using the traditional menu bar, select View|Layout|Folder Pane Columns. Or, click the toolbar menu button and then select Options|Layout|Folder Pane Columns.
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.
Tools > Options > Advanced > Show expanded columns in folder panel.
In this way you can see the size for all the folders at the same time.
The path above applies to Thunderbird 2 and earlier versions.
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.