how can i reduce the font of my header site name?

You can do that by editing the style.css file of your theme.

This is the part that tells the browser what size the header title has to be:

#header h1 {
	font-size: 3.0em;

3.0em means that it’s three times the default font size, which in your case is 10. That means a font size of 30 pixels.

You can change that for 2.0em, 2.5em or whatever you want.

Cheers!