Reply To: Media Sizes

When you upload your featured image, make sure you choose the correct size (small, medium, large).  The option will become available as soon as you upload (should be near the bottom of the popup screen).

Also, you may need to download and run a program called Regenerate Thumbnails – http://wordpress.org/extend/plugins/regenerate-thumbnails/ this will adjust all of your images to fit better in places like your slider, portfolio, etc (wherever a thumbnail/featured image is being used).  We all use this plugin anytime we make a change, or add a new images to make sure that everything adjusts immediately.