Size of header text

Are you talking about on the very top of the site, or where the logo goes?

As Erik said, what header text?
Do you mean the site title/description or maybe a header widget’s content?

I ment the site title, the very top.

In your Child Theme or CSS Editor Plugin try this code:

#site-title {
    font-size: 20px;
}

#site-description {
    font-size: 15px;
}

*Note
If your theme has a custom CSS Editor in it, you can also paste the code in there.

Never edit core CSS or PHP files, they’ll be deleted when the theme is updated.

Thanks! It’s exactly what I need, for this site and others. I cant get it right it seems though.

/*
 Theme Name:   Frontier Child
 Theme URI:    https://eindhoven-mondiaal.nl/frontier-child
 Description:  Frontier Child Theme
 Author:       Ad Camp
 Author URI:   https://eindhoven-mondiaal.nl/
 Template:     frontier
 Version:      1.0.0
 License:      GNU General Public License v2 or later
 License URI:  https://www.gnu.org/licenses/gpl-2.0.html
 Tags:         bigger title
 Text Domain:  frontier-child
*/

@import url("../frontier/style.css");

/* =place your own css here
#site-title {
    font-size: 40px;
}

#site-description {
    font-size: 30px;
}
-------------------------------------------------------------- */

Can you see a mistake? I didnt make a functions.php, is that allowed?
WP can use the frontier-child theme, but I cant see a difference, except that the header image dissapears.
Thanks for any input on this.

Can you see a mistake?

You placed your css code between this /* and */
That turned them into comments which aren’t interpreted by the browser.

/* place your own css here - this line is a comment */

#site-title {
    font-size: 40px;
}

#site-description {
    font-size: 30px;
}

Ah ok. Thanks. That did it.