The recommended amount for ranking higher in HP and Google is 500+, but this is not to say that it's a solid rule. A hub that has over 300 words can be comfortably interesting. You might want to add a few pictures and a video or two to make it a little more presentable.
If the hub has too little content it will be flagged for low quality, so research your article well so that you can achieve the high quality of over 500 words to rank higher. I would say aim for more than 300 if you can't do the 500+.
For your hub to be entered into any contest on HP the hub must have 500+ words to qualify.
Poems can have any amount of words.