Menu Font Size

I’ve been trying to change the font size for my main menu for the last couple of days. I managed to get the position and font family changed, but the same CSS class won’t affect the font-size. I’ve tried adjusting several selectors, but no success.

.main-navigation a {
font-size: xx-large;
font-family: Arial;
display: block;
float: left;
padding: 16px 16px 16px;
height: 20px;
}