Since Microsoft haven't already released this functionality in Power BI Report Server I decided to put together a bookmarklet that allows to do that.
javascript:(function(){elements = document.getElementsByClassName('pbi-frame');element = elements[0];if(element.requestFullscreen) { element.requestFullscreen(); } else if(element.mozRequestFullScreen) { element.mozRequestFullScreen(); } else if(element.webkitRequestFullscreen) { element.webkitRequestFullscreen(); } else if(element.msRequestFullscreen) { element.msRequestFullscreen(); }})();
If you have newer Chrome versions: