[SoftTalker]

With rubrics, or more simply the teacher could hand out an example essay at the start of the year that conveys the style and level of detail they are looking for when they assign an essay. Then they can refer to that when they make an assignment. Implicitly that gives a word count or number of pages, but allows for marking down for "too much repetition" or "needs more detail"

[watwut]

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.

[SoftTalker]

A good rubric would remove a lot of this ambiguity.

[watwut]

Unless it contains also minimal and maximal length, it wont. You just cant get away with not making those two clear and transparent.