How to add the option to choose between sizes for featured image within post editor?

<?php

// First, enable thumbnails if they are not enabled yet
add_theme_support( 'post-thumbnails' );

/*
 * Add image dimensions
 *
 * name
 * width
 * height
 * crop or resize (true or false)
 *
 * 99999 means very big number
 */
add_image_size( 'featured-portrait'  , 200  , 99999, false );
add_image_size( 'featured-landscape' , 99999, 200  , false );


// Next, choose names for these dimension sets
add_filter( 'image_size_names_choose', 'my_custom_sizes' );

function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'featured-portrait'  => __('Portrait, 200px wide'),
        'featured-landscape' => __('Landscape, 200px high'),
    ) );
}

You can vary width, height and crop to fit your needs.

See add_image_size() for more.