How to change the "Featured Image" size of blog post ?

You can change or add more image sizes to select from by looking in your functions.php file it will look something like this –

// Add new image sizes 
add_image_size( 'featured-img', 730, 420, TRUE );
add_image_size( 'featured-page', 341, 173, TRUE );
add_image_size( 'portfolio-thumbnail', 348, 240, TRUE );

then just copy/paste one of those lines and change the number to the sizes you want.

Also, I don’t think you can center those images by default since the Genesis Framework (not the child theme) adds the images with either an alignleft or alignright class to the featured images.

If you made your images wide enough then the text will automatically show up under the image. If you don’t then it will show up on the side since the class that is added to the image is alignleft or alignright…..This is what the Genesis Framework (not the child theme) adds to those images. One way to fix it is to follow this tutorial – http://bit.ly/1nGzUAr and that will adjust the text in the way that you need.