Hi Friend,

Try the following code:

<script language="javascript">


function function1(){
var n=document.form.name.value;
if (n.length>=5){
document.form.name.disabled=true;
}
else{
document.form.name.disabled=false;
}
}
</script>
<form name="form">
Enter Name:<input type="text" name="name" onkeypress="function1();">
</form>

Thanks