Many of my favorite stories are huge epics, but I don't want someone to post 10000 words at once.

SECONDED. I lose my place too easily if there's a huge chunk of text on the page. I like it when it's broken into chapters or parts for easier mental digestion! I would say my attention span is just a little shorter than yours, because I'm usually completely satisfied with fics that are 500 words or so in length.