I went though this question as I was setting up my first-ever touring bike last summer in preparation of my first-ever tour; a 4 week/ 80k feet climbing solo, self-contained tour from Sebastopol,CA to the North rim of the Grand Canyon via Yosemite, Death Valley and Zion National Park.

The bike came with 170s. And over the summer, for may overnight camping rides from the house, I tried 172.5 and 175s. All with the same size granny (a 26)

Even though, after 30+ years of riding, I am used to 175s on my mountain bike, and 172.5s an my road bike, I wet with 170 for touring for two reasons that make sense intellectually and experientialy.

A shorter crank requires less articulation of the knee joint (it doesn't open/close as much on each cycle). And, since a touring bike is ideally geared in a way that does not require a lot of force on the pedals, 170's are just more comfortable and, in my estimation, safer on the joints. (Now that I'm 50, that is more of a concern)

I ended up with a 22/36 low gear combo with 26" wheels. This is about 15.5 gear inches which allowed me to climb the Sierra Nevadas, etc. on a fully loaded bike without going above a conversational effort, climbing at about 3 to 4 mph at around 80rpm. Point here is that at such easy effort, crank leverage is a non-consideration.

So I'd recommend 170s from my experience.