You can read the details here. But the answer is below. Thanks Edward Escobar!

The first thing you can try doing is clicking “Scale slides up to fit display” option in the slideshow preferences.

From the top menu:  Keynote > Preferences > Slideshow

It’s the first option in the window.

Another thing you can do is set the size of the slide to the resolution of your screen.  When you create a new slideshow, from the template window at the bottom right corner, there is a dropdown menu that allows you to set the slide size.  You can choose of the sizes already listed, or customize your own.

You can find your screen resolution in: System preferences > Displays