functions.php

/** Add new image sizes */
add_image_size( 'featured-image', 700, 350, TRUE );

add_action( 'genesis_entry_header', 'single_post_featured_image', 15 );

function single_post_featured_image() {
	
	if ( ! is_singular( 'post' ) )
		return;
	
	$img = genesis_get_image( array( 'format' => 'html', 'size' => genesis_get_option( 'image_size' ), 'attr' => array( 'class' => 'featured-image' ) ) );
	printf( '<a href="%s" title="%s">%s</a>', get_permalink(), the_title_attribute( 'echo=0' ), $img );
	
}

style.css

.featured-image {
width: 700px;
height: 350px;
}