1) First of all go to your blogger dashboard and then click on layout

2) Then click on add gadget and select html/javascript

3) Copy the codes given below and paste it in your selected  html/javascript  page

<script>function go1(){if (document.selecter2.select2.options[document.selecter2.select2.selectedIndex].value != "none") {document.getElementById('main').style.fontSize=document.selecter2.select2.options[document.selecter2.select2.selectedIndex].value}}
function go2(){if (document.selecter2.select3.options[document.selecter2.select3.selectedIndex].value != "none") {document.getElementById('main').style.fontFamily=document.selecter2.select3.options[document.selecter2.select3.selectedIndex].value}}</script><form id="forma" name="selecter2" method="POST">
<select onchange="go2()" style="font-family:verdana;font-size:8pt" name="select3" size="1"><option value="Times New Roman"/>Times New Roman<option value="Arial"/>Arial<option selected value="Book Antiqua"/>Book Antiqua<option value="Bookman Old Style"/>Bookman Old Style<option value="Century Gothic"/>Century Gothic<option value="Comic Sans Ms"/>Comic Sans Ms<option value="Tahoma"/>Tahoma<option selected value="Trebuchet Ms"//>Trebuchet Ms<option value="Verdana"/>Verdana</select><select onchange="go1()" style="font-family:verdana;font-size:8pt" name="select2" size="1"><option value="8px"/>8<option value="9px"/>9<option value="10px"/>10<option value="11px"/>11<option selected value="12px"/>12<option value="14px"/>14<option value="16px"/>16<option value="18px"/>18</select></form>

4) now click on save and enjoy it