how can I tell how much data I'm storing with my sumitted forms?

Hi,

If you click on the little figure in the top right to access your account settings, a window pops up showing you your current account limits and how much is being used:

Only submission attachments take up space in your account.  The forms themselves do not.  To clear up space however, you do need to delete the submissions the files are associated with.

If there is anything else we can help you with, let us know and we will be happy to do so.