saya

問題一:
objPopup.show(screen.width/2 - 75, screen.height/2 - 80, 160, 80);
screen.width/2-75
選框離左邊距離為(螢幕寬度/2 - 75)px位置
screen.height/2 - 80
選框離上方距離為(螢幕高度/2 - 80)px位置
160
選框寬度
80
選框高度

問題二
objBody.style.margin = 10;
選寬離上下左右距離10px, (因為置中, 所以左右方會參考問題一寬度)
var strInner = "<center><select id=toLang size=19 onclick=parent.goGoogle(this.value)>";
size為選框內選項數目19個

因你只有4個, 所以若要完整顯示, 源碼要改為下面就沒問題了。

<script language="Javascript">
function goGoogle(toLang)
{
    if (objPopup) objPopup.hide();
    var newwin = top.window.open();
    if (newwin)
        newwin.location = "http://www.google.com.cn/translate?u="+top.document.location+"&langpair="+toLang+"&hl=zh-CN&ie=UTF8";
    else
        alert("弹出窗口被拦截。");
}
var objPopup = window.createPopup();
var objBody = objPopup.document.body;
objBody.style.border = "1 black solid";
objBody.style.margin = 10;
var strInner = "<center><select id=toLang size=4 onclick=parent.goGoogle(this.value)>";
strInner += "<option value=en|zh-CN>英文至中文(简体)</option>";
strInner += "<option value=zh|en>中文至英文</option>";
strInner += "<option value=zh-CN|zh-TW>中文(简体至繁体)</option>";
strInner += "<option value=zh-TW|zh-CN>中文(繁体至简体)</option>";
strInner += "</select></center>";
objBody.innerHTML = strInner;
objPopup.show(screen.width/2 - 75, screen.height/2 - 80, 150, 95);
</script>