From my experience with article marketing it appears you get better results if your articles are of a reasonable length -- at least 400 words and preferably over 500. (This applies to blogging as well.) It seems that search engines generally give more weight to longer articles, since they tend to have more information in them (which makes perfect sense, of course).
it looks like this factor has become even more influential recently because Ezine Articles increased its minimum word count from 250 to 400 several months ago. Also Go Articles, another major directory, seems to have increased its word length minimum from 250 to 400 words (though I'm not totally sure on that). I also recall Articles Base having a 250 word minimum in the past. But that's now at 350.
While it isn't hard to write articles of this length, there is still a benefit to writing shorter ones. If you just want to make one or two basic points 250 words is easily enough. Also, you can write them quickly, and thereby build backlinks in less time. And if you spread these around at various directories, then that helps with SEO. (Google likes a variety of sources and locations, remember.)
With this in mind I've been looking for article directories that clearly say you can write articles of 250 words or more. (Of course many that don't stipulate minimum length probably allow such short articles anyway, but you wouldn't know for sure.)
These include Sooper Articles, Versatile Contents, Free Articles Inc, Article Snare, New Free Article, Pro Articles Daily, That's My Niche, New Articles, Yes Articles, Article Field (300 words).
Most of the above directories have low PR, ranging from 1 to 3. However Sooper articles has a PR of 5.