pfft. i want my magic back xdPsst... Sora sucks. o.o
*the rasengan expands and then explodes blowing you back*
You're kind of thinking of the popping balloons challenge, you don't want the rasengan to expand continuously, only allow it to grow until it's about... as large as the rubber ball you were using. And then control it right there.
*holds out hand and imagines the rubber ball in my palm and creates an outline of the ball with my chakra and rotates the chakra inward*