How is the max size determined?

Using the plugin on a site that seems to have a limit of, say, 4MB but behind a CloudFlare setup with a limit of, say, 2MB… so when the first part of a large ZIP is uploaded (size is 2MB) we get a “413 Request Entity Too Large” from, it seems, CloudFlare. Now we can upload photos of about 1MB. Is there a way we can hack the plugin / force it to limit its chunk size to 1MB?