Stirrups should be wide enough so an index finger can slide down both sides of the foot. Stirrups that are too narrow can keep hold of the foot in the case of a fall.? This is particularly important with stirrups that have a rubber or leather covering on the metal arch and thus have more grip. don't just go by the size printed on the box, but measure the opening yourself.? The stirrup in the picture, with a woman?s size 9 boot, is 4?" across the metal arch but only 4?" when the rubber is included and is thus a bit too small.