var smallsize = 16, midsize=20, largesize=24;

function changeFontSize(inc)
{ 
 var p = document.getElementsByTagName('p');  
    for(n=0; n<p.length; n++) {   
     if(p[n].style.fontSize) {      
       var size = parseInt(p[n].style.fontSize.replace("px", ""));    
     } else {       var size = 20;       }   
    p[n].style.fontSize = inc;   
	}
}