More important to know the ETRTO size of the tire/rim. A 27" tire (old style) is likely an ETRTO 630mm Diameter. Then you'll want to know the width of the rim (tire off, inside width bead hood to bead hook) in milimeters. This will allow you to determine what tires widths will fit. If you want to get these measurements, I can help you figure out what will work and what won't irrespective of whether or not you're interested in Schwalbe tires or not.