@francesco: There is Custom CSS in the “Appearance => Customize => Additional CSS” box, where you have given important font size for the Heading 1 tag and Heading 2 tag which is causing issues.
Please check for the following CSS:
h1 {
color: #7A1090 !important;
font-size: 30px !important;
}
h2 {
color: #7A1090 !important;
font-size: 21px !important;
}
So, if you remove that font size from those CSS then you will get the original font size then you can adjust accordingly with the following CSS:
@media screen and (min-width: 64em) {
.singular-content-wrap .entry-title {
font-size: 56px;
}
}