If you’ve hit the size limit, you might need to think about whether you’re actively studying all of your decks. Are there limits on file sizes on AnkiWeb? - Anki FAQs
You can also check if there are other issues in your collection that are affecting the size – Constant Errors Uploading / Sync - #4 by Danika_Dakika .
And clean up any unused media to be sure you’re making good use of server space. Media - Anki Manual