If I remember correctly, the original sidecar units exported to Finland had 17T on the motor sprocket. That's unnecessary low for modern roads. 28T is standard on the trans. On my 35VDS I have 23T for solo. I do not remember what sprocket was in use when I had the sidecar on last time. I have a 21T, that I plan to use when the sidecar is reinstalled. My guess is that 19-21T is ok if the motor is in good pulling shape. There is quite a difference when you have passenger and a lot of stuff. The hills get steeper somehow... You are not going to climb Die Jungfrau anyway. Well, getting up the Alps is one problem, getting down a loaded V-model with factory anti-lock brakes in one piece is another thing.
Theory: Everything is perfectly right, but it does not work.
Practice: It works but you do not know how.
We combined theory and practice: Nothing works and nobody knows why!