I forgot I have a spare rad!
Just went and measured it.
The largest part of the barb on the rad inlet is 27 mm.
The shank OD behind the barb is 25.5, highly suggestive of being a nominal inch (25.4).
The ID of the hose should be a bit snugger than just size for size on the shank area where the clamp will squeeze down the hose.
So, no way smaller than a 26 mm adaptor male OD fitting is my thinking.
Too bad I don't have a spare unused stock hose to see what their ID is to know what Honda intended for class of fit.
Also, locating the adaptor up near the rad inlet looks like a good idea.
There's room and the hose run is straight.
More importantly, it would go where it should in terms of the overall coolant flow regime, as in after the cylinder head, past the (open)thermostat, in other words sensing the highest temp zone of the system once the 'stat is open and the system is up to temperature.
I just looked at the Koso website, their sizing goes in increments of 22, 26, 30 so trusting the 26 means shank OD, then 26 would be the size.
BUT the Koso FAQ instruction is to measure the hose ID for surety of correctly selecting.
(Nice would be a compact combi gauge to show both oil and water temps)