I only ever use a surcingle, not an actual bitting rig. I did just buy some (supposedly) pony size side reins, but they are disappointingly short. Apparently in small equine the side reins are meant to make the horse tuck his chin to his chest, and they are not meant for dressage type training, which is my choice of method. Oh well, I'm hoping that I can use them on the smaller minis, and they will be long enough for what I want.
I do have my eye on a pony size bitting rig from a different vendor, but I want it for it's parts...it has a nice open bridle, a good surcingle that I can also attach shaft loops to and drive with it the first few times I hitch...and it's got side reins--before I buy I will be checking measurements to make sure that everything is the size/length that I want. To use a bitting rig to teach the horse to tuck up into the frame that is desirable in the show ring--not my style at all.