<script>
	function countsize(obj){
		var objlength = obj.value.length;
		var sum = 0;
	    for(i=0; i < objlength; i++){
	      if ((obj.value.charCodeAt(i)>=0) && (obj.value.charCodeAt(i) <=255)){
	        sum = sum+1;
	      }else{
	        sum = sum+2;
	      }
	      if(sum >= 500) break;
	    }
		if(sum >= 500){
			alert('最多输入500个字符！');
			obj.value = obj.value.substring(0, i);
		}
	}
</script>