Nishiki Continental, Bilenky custom travel tinker, home built winter bike based on Nashbar cross frrame
According to Velocity (I asked), the Aeroheat measures 20.56 mm inside and 23.80mm outside (amazing tolerances that they specify to the nearest 10 microns, eh? ).
Sheldon Brown's site has a chart which of course doesn't show 20.56, but 19 and 21, from which I interpolated that the recommended range was 31 to 48 mm. Or thereabouts. 48 mm would be just under 1.9 inches.
He does say it is conservative, and I've ridden 1.95s for awhile with no apparent ill effects. Not Kojaks, and I don't know how Schwalbe's measurements compare to Continental's measurements.