var iFontChange = 0;

function changeTextSize(bIncrease) {
  if(bIncrease)
    iFontChange ++;
  else
    iFontChange --;
 var sNewSize = (1+(iFontChange * 0.1)) + 'em';
  // Find current document text size
  document.getElementsByTagName('body')[0].style.fontSize = sNewSize;
}