In theory, the size is limited only by your operating system and the storage space available to you, usually meaning your computer's hard disk. Do you allow Thunderbird to compact? Do you organize your mail into folders or let it all pile up in one place? Do you leave your email on the server? As a corollary to this, does the account connect using IMAP? The first challenge is to understand whether the "filled up" mailbox is on the server or, otherwise, on your own computer. I suspect you won't understand or know the answers to these questions, so a starting point would be to post your account settings here. In Thunderbird, go to '''Help|Troubleshooting Information'''. Clear the checkbox "Include account names". Click "Copy text to clipboard". In your reply to this forum, paste the information from your clipboard into your posting. Ctrl+v is a common shortcut for pasting.