Mail Merge main document

I’ve somehow converted a user’s Word document into a mail merge “main document” in the process of trying to figure out why her labels were printing in a larger font than her document was formatted in. Now when we try to use the document in a merge,we get a message saying we can’t because it’s a mail merge “main document.” How can I make this file useable in mail merge again, and why would it print in a larger font than it should? I tried saving as a different file name and copy/pasting the data into a new file – neither works. Thanks!