// the iframe of the div I need to access
var iframe = document.getElementsByTagName("iframe")[2];
var innerDoc = iframe.contentDocument || iframe.contentWindow.document;

// resize 'player' in the iframe
innerDoc.getElementById('player').width = "1000px";
innerDoc.getElementById('player').height = "650px";

Läuft in einem Userscript für diese URL:http://www.free-tv-video-online.me/player/sockshare.php?id=24DA6EAA2561FD60

Warum kommt Chrome mit diesem Fehler heraus und kann das Skript nicht ausführen ?:

Unsafe JavaScript attempt to access frame with URL http://www.sockshare.com/embed/24DA6EAA2561FD60 
from frame with URL http://www.free-tv-video-online.me/player/sockshare.php?id=24DA6EAA2561FD60. 
Domains, protocols and ports must match.

(Ich bin nur ein einfacher Javascript-Benutzer)

Endgültiger Code, vielen Dank an den Antwortenden:

// ==UserScript==
// @name       Resize
// @include    http://www.free-tv-video-online.me/player/sockshare.php*
// @include    http://www.sockshare.com/*
// ==/UserScript==

if (!(window.top === window.self)) {
    var player = document.getElementById('player');
    setSize(player);
}

function setSize(player) {
    player.style.setProperty("width", "1000px");
    player.style.setProperty("height", "650px");
}