The shaft diameter is ¾" but I'm having difficulty measuring the diameter inside the compression nut because I can only slide it clear by less than a half inch before it comes up against a key on the engine'shaft coupling.
Sure it's not 7/8"? See also Stuffing box packing - Moyer Marine Atomic 4 Community
I concur with having at least two sizes on hand. I'm after 3/16" for our 7/8" shaft (I suspect 1/4" is too snug), but it is not available locally.