The most common size string pot used is a 10" travel, which is fairly long for this application. You want to make sure that you use as much of the travel as possible to get the best resolution in your data. To do this, you can go back to a little bit of you high school geometry class and remember that the circumference of a circle (your steering shaft) is π multiplied by the shaft diameter multiplied by the number of turns from lock to lock. This will give you the travel you need to measure. As long as that is less than around 8" (the 10" travel minus the distance from the sensor to the shaft) you will be all set. If you find the sensor travel will be only a small portion of the sensor travel, you can increase the diameter of the shaft by putting a piece of rubber hose around it as you see in the picture.