I don't force paragraph length so much as I do detail. It seems that, if you require people to add some descriptions and explain what they are doing with more than simple "X does Y" sentences, the paragraph length will fall into place as it should. 

Other than that, I mostly enforce consistency within the story. So long as what you are doing can be justified in a way that isn't just "This way I win.", then I will probably be fine with it.