Hi ASCW
I would say articles can be between around 300-500 words and then have links in your resource box back to your blog or website - obviously always remembering the good content rule! If you've got longer posts on your blog or info on your website, then the articles could be trailers for these, with people finding more info when they click through from your links in your resource box. I don't know if you've heard of Caroline Middlebrook, but she's writing a great series just now on article writing, marketing etc which you might want to check out
Article Marketing For Blog Promotion | Caroline Middlebrook
Hope that's helpful
All the best
Tamsin/nudgeme