Thanks for the replies ... I see a lot of 3/4 and up at these sources and all materials, but nothing as small as I need. This particular one the hole in the hull is only 1/2" wide, and the hose feeding it looks about like a 3/8" fuel line inside dia (it's clear nylon tubing). I found this, which goes down to 1/2", but that measurement is the hose ID, definitely too big:
Here's Attwood's barbed fittings; the minimum install hole size is 1 1/8":
Stainless Steel Straight Thru-Hulls Barbed : Attwood Marine
They really cost saved on these things ... I count 17 plastic thru hull fittings on this boat ... can't tell if they are nylon or marelon I suppose, but I suspect they are nylon.
Going to get a picture of the one from the other side and I'll post that.