Whether I am teaching 5th graders or 12th, they always want to know, "How many words does it have to be? How many sentences do I have to write?"
So, at the beginning of the school year, I give them a copy of this chart and post a big print version in the classroom.
It is clear cut and the numbers can be adjusted to suit the grade.
A friend used it for 2nd graders who she had writing multi-paragraph essays by Christmas.