One simple way to classify your data is with equal intervals. For example of your data ranges from 0 to 25, then you can create 5 class 0-5, 6-10, 11-20, 21-25 and assign each range a circle size. In the style code, you will create stops for 0, 5, 10, 15, 20, 25 and assign each a size. Alternatively you can use the interval method and assign each range a size.