Hello,

The page header is a H1 heading. You can change heading styling in General Options > Typography Options. But this would change all H1 headings in your site. If you only want to change the page header heading, then do it with custom css.
Example code:

.header_page.with_subtitle.normal .titles h1 {
    font-family: sans-serif;
    font-size: 20px;
}

Let us know.

Best regards!

Hello,

Sorry i misunderstood you. To change the font size and style of Default Page header, please go to General Options > Styling Options > Default Page Header. Find the type of header you are using and change it’s font size there.

Best regards!