What's the Available Space limit of the free plan?

Hi, how many bytes does one form take up? 100MB of storage would amount to how many submissions?

The Available Space limit of the free plan is 100MB. This doesn't correspond to the submission itself, it refers to the files uploaded during submission. This would mean that this limit will only get an increase once you add a File Upload field to your form and users start uploading files along with their submissions.

And in case you'd ask, yes, you may free up some space by deleting submissions that have associated file uploads in it. However, be mindful that this process can not be undone.

Here's a snippet from the guide I'm linking below:

When deleting submissions individually, 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 

So I strongly urge you to make a backup copy of your Submission Data just in case you'd still need them. There are plenty of ways to do that, feel free pick one that will work for you:

Related guides:

How-to-export-all-of-your-data-at-once

How-to-Download-Form-Submissions-as-Excel-CSV-PDF

How-to-Download-the-Submission-Files