Your stirrups should be one inch (2.5cm) wider than the width of your boot at the ball of the foot. When your foot is in place, you should have ½ inch of space on each side. While you don't want your boot to fit too snugly into your iron, you also don't want your stirrup to be too wide.

SizeStirrup Width
1104.33"
1204.72"
1254.92"
1305.11"