That's some impressive growth

It actually exist something like a ruler for breast size?

Not at all. A ruler like this would work only if all metrics share the same under bust size (the rib cage measurement right under the breasts). Apart from trial and error, soft measure tapes and a table chart is the most common method to determine cups based on bust and under bust measurements.

EDIT: In this picture we see they compare a 70cm under bust size with a 65cm ruler, while a bit different both barely fit the range measure.