I'm in your exact same position. Oddly enough, this seems like a taboo subject here, anytime I bring it up in a thread, the thread just dies or my post is ignored.
This is what I have gathered:
- It is a good idea to send some letters of interest/update. There are a several testimonials that shorty after sending such a letter, a interview invite arrived.
- Some send by email, other by snail mail. Snail mail is more formal and benefits by the fact that it can't just be deleted, they often have to physically pull your application and file the letter, the idea is that this action will bring the applicant into the mind of the admissions committee for a little while. Downside is the letter may only be read by the mail secretary and then filled and ignored; whereas an email can be sent directly to the dean. Your choice.
- It is best to send a combination update/interest letter, compared to just an interest letter.
- It is much better to send the letter AFTER an interview. But alas, many of us don't have that luxury.
- Make sure your school accepts such letters. Some schools state on their websites "We do NOT accept interest letters". Not knowing this and sending them one would be a unwise move.
- Finally, only send ~3 letters. Don't send a letter expressing your deep interest to all 20 schools you applied to. This is very insincere and frowned upon.
I do not however have the slightest clue on how long a letter should be. There are no templates anywhere, and like I said, everyone here seems really hush-hush on the specifics.