What is the largest file size I can have uploaded through your site?

For each submission, the form can handle up to 1 GB of total file size upload. You can adjust the maximum file size by following this guide: https://www.jotform.com/help/33-Changing-the-upload-size-limit 

However, please note that each account type also has a upload storage limit. For "Free" plan, you are allowed to have a total of 100 MB of upload space. You can check our pricing page for the difference of each plan. So if your user uploaded a file of 500 MB, then your account will already exceed your limit, when you exceed this limit, all of your forms will temporarily be disabled. You need to free up your upload space by deleting submissions with file uploads on them or you can simply upgrade to a higher subscription plan with a higher upload space limit.

I hope that helps.