Most of the time people want a bigger font size to grab attention of their readers specially for the heading elements. When writing the post content, you can use different heading sizes.
In the visual editor mode, templates will have a default font applied to them. When you want to change the site font globally through out the site, you will need to edit the CSS "style.css" file for your WordPress.
This article will explain how to change the font globally for your WordPress site using the "Twenty Twelve" theme.
Note: You can change the font through out your WordPress site by following these steps for any theme not just with "Twenty Tweleve" theme.
Steps to Change the Fonts size in WordPress
Step 1) Log in to your WordPress Admin panel.
Step 2) On the left side of page you need to go to Appearence and click on Editor as showing below in the screenshot:
Step 3) Once you clicked on Editor you will see main "style.css" file of your installed theme as shown in this below screenshot.
Note: In screenshot we are editing the style.css file of our installed "twenty seventeen"theme.
Step 4) Now you need to paste the below code in the top of your"style.css" file .
Note: The (*) in the CSS code will apply font style to all text in the website. In this way we are changing the font to "courier New, Courier , monospace" font type.
Step 5) Now you need to click on Update File button to save the made changes as shown in below screenshot.
Now your Fonts are changed globally in your WordPress website. You can visit your website to check whether fonts are changed or not. You can check your website after clearing cache and cookies of your browser.