I suggest you write at least 400 words for one article according my experiences. I tried twice with ETS: at the first time, I merely got 3 on writing because I took TOEFL without any practice and had score 5 that I thought I should have no probelm with writing. Unfortunately, when the topics jumped on screen, I had no idea about it, that means, I had no examples to illustrate my idea and only wrote 300 words. Then second time, before the test, I tried to put more content in, using the same model, wrote around 400 words(maybe 380 words), I got score 5.If I had more cogent examples to write, I should have scored more than this. So, 400 words is highly-recommended.
