Noob question time: How can I download furaffinity pics in their original high resolution and not at their smaller re-size as they're uploaded at furaffinity?

You don't, Either the uploader jumps threw the hoops to get the proper res to FA or you find it on another website.
(They might give it to you if you ask, but don't use that)

Otherwise, here is a gift provided from the website for Best versions
https://e621.net/wiki/show/howto:sites_and_sources#table