I'm ground driving my colt and am thinking about getting a surcingle to keep the lines in place (my saddle isn't going to work because I don't currently have a cinch small enough for him to hold it on and my endurance stirrups have cages on them so I can't run the lines through them). I'm out of town at the moment and would like to go ahead and order one so it's there when I get home, but I'm not sure what size generally works and have never used one before. He's a VERY stocky quarter horse, but is 21 months old and only 13.1 hands at the moment. He's probably more of a typical yearling's size right now compared to the other 2 year olds I've seen. What surcingle size generally fits this age/size of horse? And, how do I measure if I don't have the change to get one before I get home?