The length of the bottom piece should be the width of your buttocks. Ideally it should also be slightly curved to fit your back. The length of the two side pieces should be equal to or slightly longer than the distance from your buttocks to the back of your head. I have heard of other ways to select the lengths but this works for me. The Iceman's pack frame was made from hazelnut and larch.
Hazelnut is a much tougher wood than willow.
The three sticks are held together by a combination of joints and lashing. Figure 1 shows detailed dimensions of the pack that works best for me. If the pack frame is too short, then a greater fraction of the weight will be carried by your shoulders. If it is too long, then the shoulder straps will be very loose and the load will wobble from side to side. My frame uses a lap joint on top and butterfly joints on the two bottom joints.