The limit on growing a mega-region with bicycles is simply that bicycles' limited speed makes them impractical for travel of more than 10 miles for most people.

But Tokyo's 38M-citizen megaregion of train stations accessed by pedestrians and cyclists[0] is probably the best-scaled megaregion yet to exist.

[0]See this random suburban station in Tokyo full of parked bicycles: https://www.google.com/maps/@35.7324121,139.4176845,3a,73.1y...