Hi, nepalidr!
So, keep in mind that my e-mail was also an update. First paragraph was just saying who I was and also reminding Ms. Walk that we had met twice previously etc. (two sentences)
Second paragraph was my update, details about the research etc (prolly 4/5 sentences)
Third paragraph was basically going back to my amcas and personal statement and highlighting why I want to go to Howard and how it is my first choice, how i'm crazy about it (2/3 sentences) etc.
Closed wishing happy holidays etc.
BUT 😱.... I talked to Ms. Walk last week to see if she got the e-mail and she basically said it was too long and she didn't read it the first time she got it (in fact, she said she was gonna reply "too long. did not read". So, I don't know what length is appropriate. But I made sure she got the information I wanted her to get and I reiterated my love for the school to her directly ....
Good luck, hope this helped!