It's nice to know what your teacher is looking for in length, because 
that tends to parallel the complexity of the ideas you will develop 
within a written work. It provides a guideline for the extent of your 
research or analysis. It can also mean that you know you only have to 
go so far, and that the essay really does not have to be an in-depth 
study, but rather a light scraping at the surface of the issues.

However, when I'm writing toward a word count, I find myself fearful 
that I will fall dreadfully short, and I begin to pepper my paper with 
unnecessary words, expanding abbreviations, and being as wordy as 
possible. I fear I will not be able to reach the assigned work count
with only my thoughts, and I want to make it. In doing so, the quality
of my writing goes down the drain. I stretch out thoughts that do not
need it, and give excessive qualifications to my every contention. I
fall victim to this even though I know that the best writing comes 
when words are applied sparingly and with great care and attention to 
detail. Because my main focus becomes that looming word count, I allow 
myself to wander in my writing, steering away from the main purpose in 
order to breach the desired number set forth for me.

On another hand, if for some reason I find myself swimming in a wealth 
of knowledge on some particular topic, a word count would force me to 
trim away excess detail and pin point the main purpose of my writing. 
This limitation could force me to examine what is truly necessary to 
the point of the essay, and leave out superfluous words and details. 
This could ultimately create a clean, well-planned, precisely written 
report on a distinct subject. However, my experience tells me that 
such occurrences are scarce.