If I am writing informational articles for websites low long should they be.

This is not copy about their products, services, about page etc. This is content articles like

1) 'What are the advantages and disadvantages of asphalt shingles vs cedar shingles'
2) 'Why clean your furnace fiilter and how often'

I was thinking 500 words but this article is about 600 words long at it seems a little short:

http://www.servicemagic.com/article....get.13634.html

So maybe I should make them 1000 words sice I want to provide usefull content as place it on other sites for backlink purposes.