Just use window.open() function? The third parameter lets you specify window size.

Example

var strWindowFeatures = "location=yes,height=570,width=520,scrollbars=yes,status=yes";
var URL = "https://www.linkedin.com/cws/share?mini=true&url=" + location.href;
var win = window.open(URL, "_blank", strWindowFeatures);

I know this reply is a little late, but I felt that a more general solution may be helpful for other people (trying to override other methods)

function wrap(object, method, wrapper){
    var fn = object[method];

    return object[method] = function(){
        return wrapper.apply(this, [fn.bind(this)].concat(
            Array.prototype.slice.call(arguments)));
    };
};

//You may want to 'unwrap' the method later 
//(setting the method back to the original)
function unwrap(object, method, orginalFn){
    object[method] = orginalFn;
};

//Any globally scoped function is considered a 'method' of the window object 
//(If you are in the browser)
wrap(window, "open", function(orginalFn){
    var originalParams = Array.prototype.slice.call(arguments, 1);
    console.log('open is being overridden');
    //Perform some logic
    //Call the original window.open with the original params
    orginalFn.apply(undefined, originalParams); 
});