If you right-click the Trash or Deleted folder, Properties, set 'Delete messages more than N days' on the Retention Policy tab.

Some user manuals:

http://www.nidelven-it.no/documentation/thunderbird/introduction-to-thunderbird

http://write.flossmanuals.net/thunderbird/about-thunderbird/

Would I be right that the emails in the deleted folder empty into the trash folder which then empties into.. nothingness?

Trash and Deleted are two names for the same folder, depending on the mail provider's naming scheme. When a message is deleted or Trash is emptied, it is marked as such and removed from view, but not permanently removed from the disc until the folder is compacted. You can compact manually from the folder right-click menu, or it will be done periodically if set to do so in Tools/Options/Advanced/Network & Disk Space.

Do folders like deleted, sent and others in Thunderbird - have a maximum size like they used to in Outlook Express (which was 2 gig I think) ? And thanks heaps for the info so far.

On 64-bit operating systems, there's no inherent limit to the size of folders imposed by TB. You're more likely to be limited (for IMAP accounts) by size limits imposed by the mail provider. You can always move mail to Local Folders if server limits come into play.

Thats good to know - thanks again