I think Cathe uses a 55 cm. ball, but I'm not sure. That's the size that is recommended for her height.
I am 5' almost 6", and I use a 55 cm height, which is what I usually see recommended for my height, though various sites give different recommendations, and on some, I'm right at the cusp of using a 65 cm. 55 cm has worked fine for me. If I had long legs, I think 65 cm would be better.