As far as I know you can't control the look of the balloons in the stand alone Google Earth application. If you are referring to the Google Earth plug-in for web pages, then you can do some slight modifications (like setting the width and height).
I have made a website where instead of a balloon opening, I prevent that event, get the contents of the balloon and show them in a dialog.
You can see my page here
http://www.3dwhistler.com/
A basic example of what I am doing is this:
google.earth.addEventListener(ge, 'balloonopening', function(event) {
event.preventDefault();
// sometimes event.preventDefault() doesn't work so use the following line instead (uncomment it obviously)
// ge.setBalloon(null);
// find out which placemark's balloon tried to open
var placemark = event.getBalloon().getFeature();
var placemark_desc = placemark.getDescription();
var placemark_name = placemark.getName();
// use this if you have 'unsafe' stuff in the balloon - eg javascript
var placemark_desc_active = placemark.getBalloonHtmlUnsafe();
// Do something with the info
});