カウトダウン機能のjavascriptの文字サイズを画面サイズによって変えたい。

下記は日付のカウントダウン機能のjavascriptのコードです。
pcサイズはフォントサイズ50pxなのですが、スマホではもう少し小さくしたいと考えています。
javascriptがわからないなりに色々いじってみたのですが、うまくいきません。
javascriptに詳しい人ご教授お願いいたします。

<head>
<title>countdown</title>
<meta http-equiv="Content-Type" content="text/html; charset=Shift_JIS">
</head>


<body style="margin:0; padding:0; text-align:center; font-weight:bold;">

<script type="text/javascript">
<!--
var nannen = 2017;
var nangatsu = 3;
var nannichi = 27;
var nanji = 23;
var nanpun = "59";
var iro = "#00469f";
var haikei = "";
var size = 50;
var haba = "";
var kaigyou = "しない";
var word = "";
function div() {
 if(iro != ""){
  iro = "color:" + iro + ";";
 }
 if(haikei != ""){
  haikei = "background-color:" + haikei + ";";
 }
 if(size != ""){
  size = "font-size:" + size + "px;";
 }
 if(haba != ""){
  haba = "width:" + haba + "px;";
 }
 document.write("<div id='timer'style='padding-top:0px;padding-bottom:0px;padding-left:5px;padding-right:5px;" + iro + haikei + size + haba + "line-height:1.1;'></div>");
}
function timer(){
 if(nangatsu == 1){
  nangatsu = "January";
 }else if(nangatsu == 2){
  nangatsu = "February";
 }else if(nangatsu == 3){
  nangatsu = "March";
 }else if(nangatsu == 4){
  nangatsu = "April";
 }else if(nangatsu == 5){
  nangatsu = "May";
 }else if(nangatsu == 6){
  nangatsu = "June";
 }else if(nangatsu == 7){
  nangatsu = "July";
 }else if(nangatsu == 8){
  nangatsu = "August";
 }else if(nangatsu == 9){
  nangatsu = "September";
 }else if(nangatsu == 10){
  nangatsu = "October";
 }else if(nangatsu == 11){
  nangatsu = "November";
 }else if(nangatsu == 12){
  nangatsu = "December";
 }
 limit_data = nangatsu + " " + nannichi + "," + nannen + " " + nanji + ":" + nanpun + ":00";
 limit = new Date(limit_data);
 today = new Date();
 days = Math.floor((limit-today)/(24*60*60*1000));
 time1 = Math.floor(((limit-today)%(24*60*60*1000))/(60*60*1000));
 time2 = Math.floor(((limit-today)%(24*60*60*1000))/(60*1000))%60;
 time3 = Math.floor(((limit-today)%(24*60*60*1000))/1000)%60%60;
 time4 = Math.floor(((limit-today)%(24*60*60*1000))/10)%100;
 if(time2 < 10){
  time2 = '0' + time2;
 }
 if(time3 < 10){
  time3 = '0' + time3;
 }
 if(time4 < 10){
  time4 = '0' + time4;
 }
 if(kaigyou == "する"){
  br = "<br>";
 }else{
  br = "";
 }
 if((limit - today) > 0){
  time = word + br + days + "日と" + time1 + "時間" + time2 + "分" + time3 + "秒" + time4;
 }else{
  time = "あとわずか";
 }
 document.all.timer.innerHTML = time;
 setTimeout('timer()',10);
}
// -->
</script><SCRIPT language=JavaScript>div();timer();</SCRIPT>

</body>
</html>