What are the key factors to consider when determining the right handlebar diameter for a mountain bikes handlebar stem, and how do manufacturers recommendations often fall short in providing a universal fit for riders of varying hand sizes and riding styles? Is it solely a matter of comfort, or are there performance implications to be considered as well? Can a rider with smaller hands still effectively control a bike with a larger diameter handlebar, or would a narrower diameter be more suitable? Conversely, would a rider with larger hands be better suited to a wider diameter handlebar, or would this compromise control and agility? How do ergonomic considerations, such as the shape and angle of the handlebars, influence the ideal diameter for a given rider? Are there any emerging trends or innovations in handlebar design that are redefining the way we think about handlebar diameter and its relationship to rider fit and performance?