Well this seems to be a common question so i think ill answer it in the most comprehensive manner incase anyone else has a Q.
The essay for dental school admissions is a personal statement, or statement of purpose. This is the applicant chance to come to life on paper. No matter how great your stats are the PS set you aside and make you all the more attractive as an applicant.
So does size matter? 😉
Sometimes. If you have a lot you want to convey then perhaps, but keep in mind that ad-coms received 1000's of applications and sometimes being selective on your subject matter can work in your favor (that doesn't mean lie about that conviction or something). I think that a good personal statement doesn;t need to be longer than one typed page. In fact, mine was exactly a page long, and very direct. I spent a good amount of time and it paid off. I know some people who wrote theirs in an hour and it didn't do anything for them, and others who labored on it and made it an epic (also didnt work for them).
In my humble opinion try to show yourself as a person who is interested in dentistry. Show them your interests, desires, pursuits, etc. Everyone who is applying to dental school with decent grades and scores will talk about how much they want to be a dentist and have dreamed about it. But what have you done to make your approach different? What as a person can you bring to the profession of dental medicine?
Remember be informative, but be precise. Following that rule: A page should suffice.
Hope that helps. Need anything PM me or ask