What size prong collar and/or head halter should i buy my gsd?

Take your dog with you when you buy it, that way you can have an istant size it properly for you. Be sure you know the proper tecnique for whichever training tool you choose. Used wrong they will do more harm than good. With halters you have to be careful the dog doesn't wrench his neck by jumping ahead and then being jerked back, be extra careful that doesn't happen.