How to change according title size

You can add CSS styles to customize the look, as with anything else on your WordPress website. How you do that depends upon your theme and WordPress setup. Some themes have a CSS editor in options. Or, developers use a child theme and edit the style.css file. Or, the JetPack plugin provides a nice CSS Stylesheet Editor.

You need to add CSS style rules, and examine the HTML classes to alter using a tool like the Google Chrome DevTools.

Here is a style rule that will enlarge the Toggle item titles by 1.5:
.omsc-toggle-title {font-size: 1.5em;}