Having spent a few, but happy, minutes with a tape measure, it seems I have been guilty of overstating the size of my equipment (it's a man thing).

Twenty inches is nearer the mark than thirty for the size of a morris napkin: the sets we had made 20 years ago were almost exactly 19", but the new pairs made last year are slightly larger, at 500mm (still just under 20"). This gives a diagonal distance of about 28", which, even with my short legs, would not quite touch the ground unless I were to dance barefoot.