How to find the right size
1) Measure the length of the old bladder. Pull the old bladder out and measure the full length.
2) Pick a new bladder that has the same length as the old bladder. Choose the longer bladder if you are not able to find a bladder that matches the length of your old bladder.
FYI - you can invert the ends of a bladder that is too long in order to make it fit.