I’m guessing you haven’t changed the width in your Slider Settings?
Once you do that, it should change. If it doesn’t change immediately, you can speed the process up by downloading and running a plugin called Regenerate Thumbnails which will automatically adjust all of your images to the size you have specified for your thumbnails throughout your theme. This is what all Genesis users use for their images.
http://wordpress.org/extend/plugins/regenerate-thumbnails/