I was thinking of writing a really long article, and I think it'll be too long to fit on one page. I was thinking of splitting it up onto multiple pages in Wordpress, but I don't know if this is a bad idea SEO-wise.
If your total HTML file size is under 100kb and your article only covers one topic, then I'd probably leave it. If the file size exceeds 100kb and/or you start discussing different topics in your article, then I'd split it into two. However, as far as the search engines are concerned, I don't think you have much to worry about.
some people split it into couple of posts and release the posts in stages, that way your visitor will be back to read the next posts. provided they like what you wrote ofcourse
Can you split your article into clear sections, like: "Article Subject - Part 1" "Article Subject - Part 2"? The when someone finds your first page, nudge them in the direction of the second part, i.e. "To read more about this subject, click here"?