I sent a letter of interest that was more than a page, and I got an interview so there is no "standard" way to do this. If you need more room, then you should use it to make your letter stronger.
Couple of pointers
1) Begin by asking them to reconsider your application for interview, take you off hold, or what ever position you are in. Say why? Briefly hit all the points of why you want to go to their school. In the subsequent paragraphs you will talk about it in greater detail
2) Each paragraph should describe ones aspect of you want to go there. Describe how it fits with your EC's which shows genuine interest rather than some unfounded statement like "I love global health".
3) After talking about why it's a great fit for you and what you would get out of a education there, talk about what they would gain from YOU. How you would contribute to say the diversity, student body, blah blah. This could be through EC's, divsere background (but don't mention grades or MCAT scores).
4) Update them on what you have been up to briefly (include grade updates).
5) Finish off strongly, summarizing the points you made before. Say that you really feel that the school would benefit greatly from you being a matriculant. And that if given a chance to speak about yourself, you are confident that you will impress them. And that you have what they want......
Hope that helps. Obviously some of this you can take out or add some new stuff on yoru own. But this is what worked for me. Hope that helps!!!!
Don't make it long just for the sake of making it long. The best length is the length where you explain everything but you don't ramble on and on.