What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
At JotForm, we want to make sure that you’re getting the online form builder help that you need. Our friendly customer support team is available 24/7.
We believe that if one user has a question, there could be more users who may have the same question. This is why many of our support forum threads are public and available to be searched and viewed. If you’d like help immediately, feel free to search for a similar question, or submit your question or concern.
You can track your usage by going to your Account's Usage page https://www.jotform.com/myaccount/usage From there, you'd see your Upload Space total.
And yes, you may free up some space by deleting Submissions. Deleting Submissions will also delete the associated file uploads in it.
Just bear in mind that when deleting submissions, you will see this warning message (see screenshot below). Please note that deleting submission data is irreversible and will permanently remove it in our servers, including the file uploads on them.
Complete guide: How-to-Delete-Form-Submission-Data