Just like that message says, you’ve reached the collection-size limit for AnkiWeb – Are there limits on file sizes on AnkiWeb? - Anki FAQs . No, there’s no way to increase that for your account.
This limit only applies to the text part of your collection – text in your notes, text in your note types/card templates, review history, etc. But 300 MB is an exceptional amount of pure text, so it’s possible there are things you can do to reduce the size of your collection. Start here for some ideas about where to look – Why is my Anki data size exploding? - #9 by Danika_Dakika .
Even though it’s not subject to that limit, you should look at your media size again too, because you’ve got over 12 GB of media in your account. Run Tools > Check Media > Delete Unused Media (and then follow that up with Empty Trash, when you’re ready) to make sure your devices and the server aren’t wasting time and space keeping unnecessary media in sync.
If you think you’ve already done that, and you’re still at 12 GB –
- If all of your devices weren’t fully in sync when you did that deletion, it’s likely that syncing restored those files for you. AnkiWeb really doesn’t want you to accidentally lose any media. Get fully synced first and then go through that process again.
- You might have some huge files contributing to that. Look for those and find ways to make them smaller.