In general I was wondering how many words should be in an article to make it search engine spider friendly? I have heard that main keyword phrases should be place once for every hundred words. Is this correct?
Are you talking about on your site? Or, article that are used in article directories? Keyword saturation is over-rated. Just make sure you have a few mentions and utilize in the Title, alt image text (for images), heading, etc. don't force anything though. If it doesn't read well because kw saturation is unnatural then it defeats the purpose of ranking well. Plus, the bulk of sites on the web rank because of their back links and anchor text.
It depends on where you wanna publish your article. There are too many opinion about keyword density. Mostly people tend to stick between 2-3%. But what I found is equally important is to use LSI keywords as well. That increases the relevancy without keyword stuffing. Hope that helps!