My recommendation for the Rowlands Flowform is a quarter of the width of the kite for the diameter of the inlet of the drogue. The ration of outlet to inlet I would say 0.25. The length of the drogue is your choice. I wonder, if a short one works better as a longer one. The distance between kite and drogue should be about 7 times of th depth of the kite.
Find more information about this topic here: