Move your arms in arches or semi-circles instead of making full circles as if you're stirring a big pot. Alternate between left-to-right arches and right-to-left arches.
Making full circles is unnecessary and awkward. Eliminating the bottom end of the circle takes away the aspect of the exercise when your arms are fully underneath you, which is the easiest portion of the exercise on your abs, but the most awkward on your shoulders.
A smaller ball makes it more difficult to perform than a larger ball because you're lower to the ground. It's just like push-ups – the higher you elevate your arms, the easier the push-up becomes because of your body angle.
If you want to mix up your set and rep ranges, you can use a smaller ball on the days when you do fewer reps and more sets, and use a larger ball on the days when you're doing higher reps and fewer sets.