Please go to Template Options -> Typography and check if heading 2 is disabled.

If it is disabled then kindly enable it and put your desired font size there and I hope it solves the problem, please do let me know.

However, if it does not solve the problem, then you can add the css rules below in Custom css section.

.article-list .article .article-header h1, .article-list .article .article-header h2 {
    font-size: 2.0rem;
    font-weight: 600;
}