To determine the length of the system to purchase, you'll need to know your static water level and well depth. It is advisable to install your hand pump for wells so that the foot-valve (water inlet end) is as far below static water level as possible to protect from draught and seasonal water table fluctuation. Doing so will also provide additional water reserve in the well while drawing from deeper, cleaner water sources. Stay 10 to 20 feet off the bottom to avoid pumping silt.
Static Water Level is the distance between the ground surface to the top of the water in the well. If you don't know this distance, there are several methods you can follow to check. One of the most common is to attach a float or weight to some fishing line and lower it slowly into the well until you hear/feel it touch water. Then, simply mark the string at the well head and pull it back out. Measure the distance between the weight and your mark to find your static water level. Be careful to use a non-lead weight and something that will not get stuck on any wires or valves down in the well.