Over a distance, on average terrain, the larger diameter wheel will always be faster, whatever you do with the cranks.
On a steady ride with no challenging obstacles, shorter cranks will tend to make you faster on a given wheel size - but there is a threshold after which the lack of control becomes counterproductive. Rule of thumb: don’t go any shorter than the length of cranks which allow you to idle for a short time.
For serious MUni, most people recommend a 24 with the fattest tyrre you can get. A 24 with a 3 inch tyre is better than a 26 with a standard tyre.
On any surface, the fatter tyre will give a smoother, more predictable ride.
For blasting along rough tracks, with no drops or tight turns (e.g. forest trails, fire roads and the like) a 29 or Coker is a good option. Big wheels roll over uneven ground better, but are harder to manoeuvre.
The 24" munis will generally have a 3" or 2.6" tire. A fat 3" tire on a 24" rim has a final diameter that is just a little bit less than a standard 26" XC MTB wheel.
The advantage to going with a 24" wheel instead of a 26" wheel is maneuverability. In technical areas you want more maneuverability. For example when riding through a rock garden or through a maze of tree roots or just riding along a tight trail. With a 24" wheel your feet are in the stable position (pedals horizontal) more often. You’re in a good pedal position to turn and maneuver more often.
A 26" rim with a fat tire is pretty big. A 26" rim with a fat 3" tire has a final diameter that is close to 28" inches, almost as big as a 29er. Not exactly a nimble little unicycle. A 26" wheel with a fat tire is also going to be heavier than a 24" wheel with a fat tire. It is easier to jump and hop with a 24" than a 26" due to the lower weight and smaller wheel size.
If you’re doing technical riding a 24" wheel just works out better. For non-technical riding where speed is the goal then a 29er would be the choice. A 26" with a fat tire would be more for aggressive riding that is too much for a 29er but not tight and technical and something where you’re willing to deal with the extra weight.