Yeah, that number changes each time you re-publish the page. It'll be tricky to do what you're after.. not sure if anyone else has a way to do this, but if it was me, I'd probably end up having to use a bit of jQuery to find an element that only exists in the widget you want to resize, and use .parent() a few times to go up the DOM to the widget frame, and set the CSS that way.
Hmm, or maybe investigate the nth selector in CSS.. if your widget is in the same position, you could set it that way.
Genius! The nth selector worked - thanks a million!
For those wanting to do the same thing: