Bilateral transtibial/transfemoral amputee!

As you see from the picture above is a bilateral transtibial amputee,how the Prosthetist will get the lenth of the prosthesis?to avoid having too long or too short prosthesis since this will bring a direct impact in Centre of Gravity during gait.So please if you have the idea on how to determine the actual length lets share with other in this village.