This style of stirrup may also be referred to as a German or Heavy Dressage Iron, or Knife Edge Iron. It is similar to the regular 2 bar stirrup, but has a much heavier base and tall, streamlined sides. The extra weight makes it particularly popular/suited for Dressage, where the stirrups should be as still as possible, but it is also suited to other types of competition riding.
To correctly gauge the correct stirrup size, the rider should place the ball of their foot on the stirrup bars, close to the inside, leaving a space of 25mm (1") to the outside.
To correctly gauge the correct stirrup size, the rider should place the ball of their foot on the stirrup bars, close to the inside, leaving a space of 25mm (1") to the outside.