In this case, I think the decision to use a 15" midwoofer was a system-design choice.
One of the priorities Dutch & Dutch has embraced is good radiation pattern behavior in the crossover region, wherein the patterns of the woofer and of the waveguide are sufficiently similar to blend smoothly. So the midwoofer's diameter was chosen to work well with the waveguide's radiation pattern; and the waveguide's dimensions and radiation pattern were chosen to work well with a 15" midwoofer.
To the best of my knowledge JBL was the first to match up the radiation patterns of 15" woofer and similar-sized horn (constant-directivity in both the horizontal and vertical dimensions) in their landmark Model 4430 studio monitor four-and-a-half decades ago. Earl Geddes significantly refined the concept by introducing the first low-coloration constant-directivity waveguides in his (imo magnificent) Summa. My guess is that when measurements become available we'll see that the Dutch & Dutch 15C is an even further refinement, with the midwoofer's cardioid loading extending pattern control down considerably lower.
