Hi there!
You can use .social-icons as custom selector. Please follow the screenshot below:

Alternative solution-
If you wnat to use custom CSS then here it is:
ul.social-icons>li {
font-size: 15px;
}
In your case section ID need to be added too. The following blog post may help you to find selectors:
https://www.joomshaper.com/blog/which-line-of-css-i-have-to-edit-code-inspector-in-action
Note: Path towards custom CSS: Extensions>Templates>Styles> Click on your current default template name then choose Template Options. Then find custom CSS within the custom code.
Best Regards