The frame size ? Does it have a horizontal top tube ? If so, you can measure "standover height" from the top tube to the ground.
Also, bikes of that era were usually measured in centimeters.... the distance from the center of the crank, up the seat tube. to the intersection with the top tube.

Once sloping top tubes became the fashion, that measurement method didn't really work any more. Some were measured to the imaginary point where the seat tube would have intersected a horizontal top tube.