Limit of words? There is no limitation of words on forums as well as articles. To earn 5 points you post should have at least 50 letters.
Regarding article, number of words may increase your credits!
As Sajeetharan told you will not get points for forum answer if it is too short.In Articles if you write one having more than 1000 words you may be paid well.
Although there is no maximum limit on words in forums, it is desirable that the posts should not be lengthy. These should be brief and concise so that the readers may get the idea just by glancing through. The discussion forums are for quick expression of thoughts. If you have to write more details and write about 200 hundred words or more, writing article will be better. I feel that a forum post should be from twenty to fifty words.