The ambiguous "needs more detail" thing would lead to a lot of students making it too brief in good faith, too long in good faith and both be frustrated and angry. You can write really good mini essay on a topic. And you can write really good super long essay on the same topic.

Demanding that students mind read is not a good strategy. Specifying expected length, checking for it is a good strategy. Teacher should also check for other things - whether paragraphs logically follow, grammar, sentence structure, you name it. But dont make them guess.