I'm often asked by clients how far or long to reach a leg or arm during an exercise, and that's where my often spoken, "Range is determined by form" comes from.
It's most important to keep your critical connections in Pilates - so if by reaching your arm you cannot keep your ribs to your scapula and your scapula to your ribs, you've gone too far. If you don't feel your three anchors, your legs have dipped too low.
But how do you determine where this end range is? It can be different from day to day, and hopefully with improve with time as you commit to your Pilates practice. In general, stay where you are working in your Powerhouse, and let that connection build in a smaller range of motion before taking it bigger. Start at the center, at the Powerhouse, not in your arms or legs. You should feel your arms and legs lengthening out from your Powerhouse connection. Stop when you loose that connection.
My first Pilates teacher made me make the tiniest One Leg Circles I had ever performed, but when I did my hip stopped clicking. I kept working on my Powerhouse connection and one day she said, "I think you can make them bigger now." But it was important for me to keep them small so that the larger motion didn't distract me from my Powerhouse. Momentum is not a muscle group, and it certainly isn't one of the Pilates Principles.
So stay inside the exercise and your body. Really listen to see if you are finding your connection. And don't be afraid to make the movement small and controlled (that one is a Pilates principle)!