Cabin baggage on Ryanair

I'm going to take Ryan flight. My bag is 21.5cm × 35cm × 45cm just a little bit bigger than the allowance of 20 cm while other company's allowance is mostly 25cm.

The bag is hard that it can't be shrunken. The box to check the size before boarding has 2cm of extra width ? Or it is exactly 20 cm ?