My letter of intent for an MSc program is only 1000 characters (which is about 160 words). I was planning to begin with a short story about how I became interested in the field, how my academic background has prepared me, my current research interests, and my future goals. All of that is about 400-500 words (I already wrote it out). I will be evaluated on how well my interests align with the school’s and my supervisor’s. So should I only include my research interests? So there really wouldn’t be any introductry line... I would just get into it?
Carefully address the question as phrased by the institution. Different schools may describe what they want as a letter of intent in slightly different ways that may help you understand what they're looking for.
Broadly speaking, a letter of intent is not about your past. It should explain your intentions for the future. (Stuff about your past is probably better in a personal statement, if you're asked for one.) A letter of intent should answer the question, what will you do if you come here?
In any sort of admissions or employment situation, whenever you're asked about the future, a good answer usually includes some specificity about your objectives, something testable, what you'd like to work on, how you think you might proceed toward your goals, etc. Hope that's helpful.