function playStart(idx,sz){
 var win = window.open ('', 'formpop','scrollbars=0,resizable=no,width=1,height=1,top=0,left=0');
 document.frm2.idx.value = idx;
 document.frm2.size.value = sz;
 document.frm2.method = "post";
 document.frm2.action = "tset.php";
 document.frm2.submit();
 win.focus();
}

# html

<form method=post target="formpop" name="frm2">
   <input type="hidden" name="idx">
   <input type="hidden" name="size">
</form>