Ryanair - Hand Luggage Size?

I'm going to Croatia with Ryanair for a week in two days and i'm just worried about my hand luggage size. I have measured it over and over again and the size seems to be okay. But i'm just wondering, what happens if my hand luggage size bag is oversized by a small amount (2cm)? Do i get charged but still able to go on board? Am i not able to go on board?