<body onload='fix("h1")'>
<style>
h1 {
font-size: 40px;
}
</style>
<script>
function fix(tagname) {
tegi=document.getElementsByTagName(tagname);
for(i=0;i<tegi.length;i++){
if (tegi[i].innerHTML.length > 20)
tegi[i].style.fontSize = '20px';
else if (tegi[i].innerHTML.length > 10)
tegi[i].style.fontSize = '30px';
}}
</script>
<h1>I'm fixed</h1>
<h1>Fix me pleaseeeeeeeeeeeeeeeeeeeeeeeee</h1></body>
можно так=)