change the defaults from outside of plugin

My plugin is :

$.fn.iPopup = function() {
     var opts = new function() {
         this.width = 957;
         this.height = 590;
         this.left = ($(document).width() / 2) - (this.width/2) - (17/2);
         this.top = 160; 
     }
     alert (opts.width);
 }
 

I want change the width or height when I call plugin , like:

$('div#tP').iPopup({width:280});
 

what should I do in my plugin?

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