Where can I find how much space I have left?

You can find this information in the Account Usage section. Here's the link: https://www.jotform.com/myaccount/usage.

Also, you need to delete the submissions with attachments to clear your storage space. Here's a guide: https://www.jotform.com/help/377-How-to-Delete-Form-Submission-Data

Hope that helps. Thank you.