Use your judgement for the sprocket size. 20" is the stock size rear tire. If you feel that you're shifting too much, and have the power to pull some slightly higher gearing, then go ahead and try maybe a 14 tooth front sprocket. I wouldn't go any lower than the stock 13/38 setup though.
22-7-10's are a better front tire size, or even 21-7-10. I have 22-8-10's on mine, and as you can see in any of my pictures, they're wider than you need on a sport quad. A narrower tire will weigh less, have lighter steering, and less bump steer over rough terrain.