This is a great question that I'm not sure the answer to. Scaling stem length to height seems like a good idea, intuitively. A bike for a bigger person should ideally have a longer reach, longer front center, longer chainstays, wider bars, and taller saddle, so having a longer stem seems to make sense. But maybe stem length is more related to steering feel, like head angle or fork offset, and should not vary with different sizes? All I know for sure is that I'm tall, I've been happy on 45's, 50's, and 60mm stems, and a 35 or 40mm...