What is the right size for an excercise ball?

You are right on the cusp.

55 cm – 4’11” – 5’4”
65 cm – 5’5” – 5’11”
75 cm – 6’0” – 6’ 7”

Choose the ball which is right for you by sitting on the ball and making sure your hips are level or just slightly higher than the knees

I’m 5’8’’ and I use either the 65 or 75 cm ball. I feel more comfortable with the 65 cm. Try the first two out and see which feels better.