Long and short articles can work for seo purposes as long as the content is relevant with enough related keywords in it to define the topic. 500 words is considered a good minimum and do for backlink building but in terms of building your own sites you should try to write more than 1000 words per page.
If you think about it, the search engine scan a page to find topic based keywords that can be clumped together to form the overall relevance of the content. The google algorithm is a program after all and not a human. The more content you have the more validating keywords will be in the content. This also means that you have a chance for ranking more keywords per page.
You won't get penalized for short content but you may get rewarded for long - no guarantees, just use your common sense. :)