We haven't had anyone yet archive a story and reach a maximum word length, so in theory, I'd say it would be fine to do as a one-shot. However, I would say that your story is so LONG, people can rarely read it in one sitting, so breaking it up into parts would make some sense. Readers would have an easier time remembering & bookmarking where they left off. Of course, that's just my opinion. :)
I'm so happy you are considering archiving with us first. We are kind to virgins of all kinds at WTP. ;) Definitely check out the screenshots. The first time I uploaded, I left it open in a window, and worked in the other one. It really helped.