For me it's mostly how far I've gotten and able to proofread before posting. Many stories are works in development, something is ready to post when I think so. I also count pages and try to make each chapter similar in length.
Sometimes I like ending a chapter on a cliff-hanger hoping people will keep looking for more to find out what happens.
As far as length, if someone wants to post a long story as a single post, that's fine. However, I have seen at least one story on another site that is just so long that it got ridiculous. I also am not a big fan of one paragraph at a time....that's even more tiresome, and I lose interest. Most of my story posts are about 2-3 pages in MS Word, but that's just me.