Where to edit the font size of the siderbar?

Depends on the Theme you are using and what in the sidebar you want changed. Could be wp-content/themes/your_theme_name/style.css.

For starters see:
Playing_With_Fonts

I know how to change codes but what I failed to do is I don’t which code is refering to the sidebar LINK fonts. I am using the default template.

The sidebar header fonts are described in line 107 of the default template. The sidebar link fonts are inherited.

Thanks for you reply ??