Jim,
I found my receipt for my PSS dripless seal. Is that what you are referring to? Otherwise, Stu is correct...its 3/16 packing. Mind you, I purchased my PSS in 1999 from West Marine and upon checking, the part numbers are slightly different. My part number was 01-100-112. The seal has a slightly different design now with a vent that mine does not have. I have to burp it whenever we haul the boat. The new ones look self burping. The new part number I see on WM is 02-100-112. This leads me to believe that I had a 1 1/2" stern tube that the bellows fits on. However, you should probably verify your own dimensions before ordering. We have been quite happy with the dripless seal. Our bilge is dry and the boat remains fresh smelling.
Allan