Hi there,
in Customizer > Additional CSS there is this CSS:
/* home hero headline */
.hero-header {
font-size: 70px;
line-height: 1.3em;
text-shadow: 1px 1px 2px rgba(20, 20, 20, 0.4);
}
/* home hero mobile headline */
@media (max-width: 768px) {
.hero-header {
font-size: 50px;
line-height: 1.1em;
}
}
This is currently setting the desktop and mobile font sizes.
You can either edit this CSS to suit. Or if you want the header element to respect the customizer sizes then just remove or comment out that CSS.