There is no upper limit, although managing such large files puts a strain on the PC resources. I suggest checking within profile to see if there are entries that can be removed, such as any 'nstmp files'. I suggest NOT zipping the backups unless you do this when Thunderbird is not running. I state that because attempts to import/restore zipped files is a road of failures. If the account is Gmail, I suggest NOT subscribing to AllMail or Import folders, as those are duplicates of other folders and increase disk space and computer resources to send and receive mail. If you want to reduce backup storage space, just exit thunderbird, delete the message folders prior to the backup. On a restore, THunderbird automatically reconstructs them. My personal approach is periodically purging unneeded messages. That does a lot to keep the folders smaller. Good luck.