var min=9;
var max=15;
<!-- vergroten -->
function increaseFontSize() {
   var p = document.getElementsByTagName('body');
   for(i=0;i<p.length;i++) {
      
      var size = cookiejar.fetch("size");
      if(size != null)
      { 
      var s = parseInt(cookiejar.fetch("size"));
      }
      else
      {
      var s = 12;
      }
      if(s!=max) {
         s += 1;
      }
      p[i].style.fontSize = s+"px"
      SetCookie('size',s,365);
   }
}
<!-- verkleinen -->
function decreaseFontSize() {
   var p = document.getElementsByTagName('body');
   for(i=0;i<p.length;i++) {
      var size = cookiejar.fetch("size");
      if(size != null)
      { 
      var s = parseInt(cookiejar.fetch("size"));
      }
      else
      {
      var s = 12;
      }
      if(s!=min) {
         s -= 1;
      }
      p[i].style.fontSize = s+"px"
      SetCookie('size',s,365);
   }   
}

function SetCookie(cookieName,cookieValue,nDays) {
 var today = new Date();
 var expire = new Date();
 if (nDays==null || nDays==0) nDays=1;
 expire.setTime(today.getTime() + 3600000*24*nDays);
 document.cookie = cookieName+"="+escape(cookieValue)
                 + ";expires="+expire.toGMTString();
}