Suppose you have this site title text and need to change the size of “ASC”

#1. First, use this code to Code Injection > Footer

<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.7.0/jquery.min.js"></script>
<script>
$(document).ready(function(){
    $("a#site-title").each(function() {
    $(this).html($(this).html().replace(/ASC/g, "<span>ASC</span>"));
});
});
</script>

#2. Use this code to Custom CSS box

a#site-title span {
    font-size: 14px;
}

#3. Result