1.Log in to your dashboard--> layout- -> Edit HTML

2.Click on "Expand Widget Templates"

3.Scroll down to where you see this :

]]></b:skin>

4.Add below code before  ]]></b:skin>  tag.

.textresize { padding-left:20px; padding-top:5px; }

5.Now add below code between  ]]></b:skin>  and  </head>  tags.

<script language='JavaScript' type='text/javascript'> function changeFontSize(inc) { var p = document.getElementsByTagName(&#39;p&#39;); for(n=0; n&lt;p.length; n++) { if(p[n].style.fontSize) { var size = parseInt(p[n].style.fontSize.replace(&quot;px&quot;, &quot;&quot;)); } else { var size = 12; } p[n].style.fontSize = size+inc + &#39;px&#39;; } } </script>

6.Now again scroll down to where you see this :

<div class='post-header-line-1'/>

7.Add below code immediately after above line.

<span class='textresize'> Text Size :<a href='javascript:changeFontSize(1)'> [+]</a> | <a href='javascript:changeFontSize(-1)'> [-]</a> </span>

8.Now save your template and you are done.