Hmm, I guess it's just the normal stirrup length that people use when they're doing dressage, so the leathers are pretty long but not so long that your feet fall out the stirrups. As opposed to jumping position stirrups that are shorter :)
Whatever is comfertable for you right now. Then slowly lower the stirrup until you feel comfertable atthat length. You don't need super long stirrups if you arnt physically ready for it. Mine hit the bitten sole of my boot when my legs are hanging down, but I've also ridden in a dressage saddle and done dressage for the past 2 years. Posted via Mobile Device
You need to ride with your irons short enough that your legs are not swinging. As your legs get stronger, you can lengthen your irons very gradually until your leg is very long.