A spray skirt is a waterproof garment that is worn over the top of a kayak to keep water from entering the cockpit. It typically has an elastic band or drawstring that goes around the waist, and a tunnel that fits over the kayak's coaming (the raised lip around the edge of the cockpit).
There are two main ways to measure for a spray skirt:
1) Take your waist measurement at its widest point, and add 6-10 inches. This will give you the minimum amount of fabric you need in order to make a comfortable, functional spray skirt.
2) Another way to measure for a spray skirt is to take your kayak out on the water without one, and mark where the waves are coming into the cockpit. Then, measure from this point down to where you want the bottom of the skirt to sit (typically about mid-thigh). This method will give you a more customized fit.
Once you have your measurements, choose a material based on what conditions you'll be paddling in most often. For example, neoprene is best for colder weather as it provides more insulation, while nylon is more breathable and better suited for warmer temperatures.