Hi,

Please try add more this code to Customize > Custom CSS to do that:

.widget ul.side-newsfeed li.featured-news .side-item .side-item-text h4 a{ font-size: 26px; }
.widget ul.side-newsfeed li .side-item .side-item-text h4 a{ font-size: 20px; }
.widget ul.penci-feed-2columns li .side-item .side-item-text h4 a{ font-size: 18px; }

This code will help you change the font size for "Recent Posts" on the right sidebar and footer widget "POST PIÙ POPOLARI"

Best Regards,

PenciDesign