I think I solved this based on a thread on the Schwinn Bike Forum (which I can't link due to my newbie status). A user JenniferC posted the following:
"Made by Giant as Bartcycle said, used a three piece Sugino steel crankset and chainring until the later 80's models when they went to an alloy crankset. They're OK bikes, your basic three speed. Blue was the basic color but later they made them in white. One or two years they made a pinkish color women's model called Dusty Rose. They're $75-100 bikes in good shape. Basic wheels for beating around. They used 26" 590mm tires instead of the Schwinn S-6 597mm tires."
So I think I have my answer... 590mm. Hurray!