[EricH]

As a general thing, obstacle traversing is better with a larger wheel (larger wheel, larger obstacle). However, larger wheels can easily be more weight, and a smaller wheel can be made to climb as effectively as a large one. Pick your poison; I'm not quite sure whether there's enough difference between a 6" and 8" or between a 4" and a 6" to make it worthwhile. You only get about an extra inch of vertical climbing ability. 4" to 8", probably enough to make it worthwhile.

On width, ideal physics says no difference, while reality says wider is better. I'll let the idealists and the realists argue that one out, preferably by testing.

Gearing matched: Given that the gearing is matched to give the same robot speed, a smaller wheel will tend to need a smaller reduction (less weight) to go as fast as a larger wheel. (Smaller reduction = higher rotational speed, for those that might be confused.)