Hi,

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

@media only screen and (min-width: 769px){
.penci-page-header h1 {
    font-size: 50px;
}
}
@media only screen and (max-width: 768px){
.penci-page-header h1 {
    font-size: 24px;
}
}

Best Regards,

PenciDesign