This js code should expand the player to the width/height of the browser window.

fullscreen-hbo.js

window._fs__hbo_timeout;
window._fs__hbo = function(){
  document.querySelector("#ifp").parentNode.setAttribute("style", "position:fixed;top:0px;left:0px;width:100vw;height:100vh;background-color:#000;z-index:2147483648;");
  document.querySelector("#ifp").parentNode.setAttribute("id", "flvideo");
  document.querySelector("#ifp").parentNode.parentNode.parentNode.parentNode.parentNode.parentNode.style.top = "0px";
  document.querySelectorAll(".default").forEach(function(e){
    var style = window.getComputedStyle(e);
    if(e.getAttribute("id") != "flvideo" && style.position == "fixed"){ e.style.opacity = 0; e.style.zIndex = 1; }
  });
}
window._fs__hbo();
window.addEventListener('resize', function(){
  clearTimeout(window._fs__hbo_timeout);
  window._fs__hbo_timeout = setTimeout(window._fs__hbo, 2000);
});

fullscreen-hbo.md

HBO GO Fullscreen

When watching on desktop at play.hbogo.com. This js code should expand the player to the width/height of the browser window.

Open Developer Tools > Javascript Console, paste the javascript and press enter.

fullscreen-hbo.min.js

window._fs__hbo_timeout;window._fs__hbo=function(){document.querySelector("#ifp").parentNode.setAttribute("style","position:fixed;top:0px;left:0px;width:100vw;height:100vh;background-color:#000;z-index:2147483648;");document.querySelector("#ifp").parentNode.setAttribute("id","flvideo");document.querySelector("#ifp").parentNode.parentNode.parentNode.parentNode.parentNode.parentNode.style.top="0px";document.querySelectorAll(".default").forEach(function(e){var style=window.getComputedStyle(e);if(e.getAttribute("id")!="flvideo"&&style.position=="fixed"){e.style.opacity=0;e.style.zIndex=1}})};window._fs__hbo();window.addEventListener("resize",function(){clearTimeout(window._fs__hbo_timeout);window._fs__hbo_timeout=setTimeout(window._fs__hbo,2e3)});