At the moment I'd like to keep it at just a single flat fee: $10 for a review. (It's only $5 right now for people beta testing)
No matter what the size. So if you have a short doc you'll get lots of edits, and a long doc will get a good look and some comments on how to improve but not a detailed proof read.
I like keeping it that way so you can hit the button and always know what to expect. You don't have to worry "how much is this one going to cost me now". You'll just know.
I see a lot of other folks charging per word. I might play with that if folks don't like the flat fee.
Reviewers currently only see the original. I don't have a feedback mechanism in place yet for the reviewers to see how their changes are getting accepted. It's on the todo list.