In most browsers, the following would work.
window.onload = function(){
console.log( document.getElementById('svgElm').getBoundingClientRect().width );
};
Here is a demo. If you try it in Google Chrome, the console will output 200. However, FireFox returns 0.
I’ve ended up falling back to the parent dimensions if SVG properties cannot be returned. Here is a demo http://jsbin.com/uzoyik/1/edit.
The relavent code is: