Upon googling, it appears that you can find Schwalbe Stelvio tires in both 25-559 (1 x 26") and 28-559 (1 1/8 x 26"). It seemed that most places that sell them deal in recumbent goods, which makes sense, since many recumbents have a rear 26" wheel.
The full wheel diameter with a 1.5" tire is just over 25". The amount a tire adds to the wheel radius is usually the same as its width. The minimum width tire you can put on is approximately the same width as your rim.