How do I change the font size of the top menu on my website?

To Whom it May Concern,

I am trying to change the font size of the main menu on the top of my website. I have been trying to use this bit of CSS to change it:

#top-menu a {
font-size: 16px;

But thus far it has not been helping. Can you offer me any assistance on how to decrease the size of the font used on the top main menu on my website?

Thank you in advance for your time.