As long as the letter is strong, the length should not matter as much. When I evaluated applications for the UC Davis School of Medicine Postbaccalaureate Program, I dreaded reading letters of recommendation that were over three pages long. I once received a letter of rec that was seven pages long! In all honestly, strong letters that are on the shorter side are more likely to be read word for word. Longer letters are not necessarily better.
*The best letters of rec are written by people who know you well and can provide honest and positive feedback on the quality of your work.*
Did you give your letter writer an updated copy of your resume/CV, personal statement and bullet points with information relevant to the course or research you completed with her/him? Often providing your letter writer with all the material that they need will ensure that you will get a strong letter of support and that they will have enough material to include about you.
I'm including a link to an article on
"The Best Medical School Recommendations."