<input size=1>
<script>
document.getElementsByTagName("input")[0].onkeyup = function () {
    this.size = this.value.length || 1;
};
</script>

...document.getElementsByTagName("input")[`x`].onkeyup...

<script type="text/javascript">
window.onload = function(){
    input = document.getElementsByName("test"), i = input.length;
    while(i--){
        input[i].onkeyup = function(){
            this.size = this.value.length || 1;
        };
    }
};
</script>
<input name="test" type="text" value="text1" />
<input name="test" type="text" value="text2" />
<input name="test" type="text" value="text3" />