Spray skirts have two dimensions you need to be aware of: one is deck size (cockpit) and the other is tunnel size (paddler's torso). It's important that the deck size match the boat and the tunnel size match the paddler.
We have used a Snapdragon Ocean Trek extra large spray skirt and a Harmony 38-20 with our Pakayaks and both match our deck size and work perfectly. By using those sizes you will be able to convert to almost any brand on the market.
For Seals skirts and covers, the sizing is a 2.2. View their fit guide here.
We suggest you to determine what type of spray skirt best fits your needs and then try them on to determine the best fit around your torso. The “tunnel” should be water-tight yet not restrict breathing while under exertion. It's best to go try one on and see which fits/feels the best on you in terms of comfort.
Here is a fit guide that REI published.