Ooooo I think one paragraph is plenty. The ones I get are usually not any longer.
Dear X,
Thank you so much for taking the time to interview me for XSOM. I really enjoyed hearing your take on the special thing you're interested in. I feel like your school is a great fit for me because XYZ. Thank you again for your time and effort!
Quick Answer: A post-interview thank you letter for medical school should be sent within 24 hours as a PDF attachment. Keep it under half a page, thank