I googled this and got a quirksmode site that gives you your screen size.  Pulling up the console I see that screen.width and screen.height are directl         
        
Take a look at Get the device width in javascript
Media Queries work in js too:
if (window.matchMedia('screen and (max-width: 768px)').matches) {}
Another way would be:
var width = Math.max(document.documentElement.clientWidth, window.innerWidth || 0);
                                                                        This is what worked for me after long hunch. These attributes below provide responsive sizes based on new tab size.
window.innerWidth
window.innerHeight
                                                                        screen.width is the property your are looking for.