To confirm the trash folder, I suggest deleting a message and see what happens to it. if it goes to your deleted folder, that is the trash folder. there should be a trash or delete in left sidebar and a right click should offer option to 'empty trash.' After emptying trash, you could do this to verify what folders you have:

1. click help>troubleshootinginformaiton
2. search for 'profile folder' in left side and then click 'open folder'
3. exit thunderbird
4. you are now in profile. click the imapmail folder if imap account or Mail folder if POP. click the account name and the various message folders and their sizes should appear. From here, you can see the sizes of all your folders and if there is a trash folder.
5. another consideration is to click Tools>clear recent history to ensure cache is clear as you don't want to save that either.
6. if imap, I also suggest rightclicking accountname in main screen and clicking 'subscribe' to see what folders you are using.
7. TIP: if imap and if you have multi-megabytes of data, you can delete the message files in windows file explorer when thunderbird is not running to save copying them, as thunderbird will repopulate when restarted.

Thanks David for your prompt response.

Sadly I'm not too computer literate. In your step 4 after clicking the imapmail folder where do I find the account name to click on to access the message folders and sizes?

Regards, Andrew

In the imapmail folder, any folder there is an imap account. Click it to see its sub folder and then you will see the message folders, such as inbox, sent, etc.