<script language="Jscript">
function chksize(strFileName){
    var maxsize = 100;    //定义允许文件的大小，单位KB，请根据需要自行修改！
    var objStream = new ActiveXObject("ADODB.Stream");
    objStream.Type = 1;
    objStream.Open();
    objStream.LoadFromFile(strFileName);
    if(Math.round(objStream.Size/1024,2)>maxsize)
        {alert ("抱歉！您选择的文件为 "+Math.round(objStream.Size/1024,2)+" KB　n超过了程序"+maxsize+" KB 的限制！");
        document.form1.reset();
        return false;
        }
}
</script>
<form name="form1" action="" method="post">
<input type="file" name="Upfile" >
</form>