Hi there,

You can add the following CSS code to Appearance -> Customize -> Additional CSS:

/* Change slide headings text size */
@media (min-width: 992px) {
    .jumbotron .jumbotron-caption .caption-small-heading {
        font-size: 15px;
    }
    .jumbotron .jumbotron-caption .caption-heading .caption-heading-tag {
        font-size: 40px;
    }
}

The 15px value can be changed to change the small heading size of the slides.

The 40px value can be changed for the bigger heading size of the slides.

Hope this helps.

Great! You're welcome

Let me know if you have any other questions.