It's not so much that Wilbur has a preference for a particular resolution. It's more that the kind of model in use generates results that I think appear truly plausible only on roughly that scale. Note that Wilbur is missing huge kinds of effects that would appear on the micro scale such as meanders or other effects that have to do with density of flow and with mass wasting (critical physical processes for real-world erosion). Wilbur's precipiton thingy is the simplest algorithm that I could get to generate something that looked reasonably plausible to me. Incise flow is just as bad in terms of having a physical meaning...