Perhaps. But notice that we’d have to have similar tags for at least bicycle:scale=*, foot:scale=* and hiking:scale=*. We need all the different modes to be present as none of these passable values for the different modes are immediately and truth-preservingly inferable from each other.
Indeed, your point about handlebar length is spot on! A path that is too narrow (wrt. the handlebars) for a normal bicycle might be ‘passable’ for a mountain bike, and a path too narrow for a mountain bike (wrt. the handlebars) might be perfectly fine for strolling with leather-soled Oxfords. And this was just about the width of the path. Similar considerations apply for e.g. incline, etc., etc., etc. Like I said, at this point, were simply duplicating a few descriptive tags with a needlessly contrived, somewhat subjective (as @tekim noted) and mode-specific tagging system.