How to change the font size of slider text in Hestia

To change the font size of the slider text in Hestia add the following CSS in Appearance > Customize > Additional CSS section.

.carousel .hestia-title {
    font-size: 70px;
}
.carousel span.sub-title {
    font-size: 30px;
}
/** CSS for mobile devices **/
@media (max-width: 768px){
.carousel .hestia-title, h1.hestia-title{
    font-size: 36px;
}
.carousel span.sub-title {
    font-size: 20px;
}
}

You can change the size according to your requirements.