Set headline font size based on the length of a headline

change-font-size-by-headline-length.css

/* This is the default headline size. */
.headline,
.headline--normal {
    font-size: 2em;
}
/* When the headline is shorter, make the font-size larger */
.headline--short {
    font-size: 2.5em;
}
/* When the headline is longer, make the font-size smaller */
.headline--long {
    font-size: 1.66em;
}
.headline--longer {
    font-size: 1.33em;
}

change-font-size-by-headline-length.php

<?php
// Store the page title in a variable
$headline = get_the_title();
$hLength =  strlen($headline);

// Set a class name for each range of character length
if ( $hLength <= 25 ) :
    $headline_class = 'headline--short';
elseif ( $hLength > 25 && $hLength < 40) :
    $headline_class = 'headline--normal';
elseif ( $hLength >= 40 && $hLength < 65 ) :
    $headline_class = 'headline--long';
elseif ( $hLength >= 65) :
    $headline_class = 'headline--longer';
endif;
?>

<h1 class="headline <?php echo $headline_class; ?>">
    <?php echo $headline; ?>
</h1>