How do I set the featured image size on the single post?

You can set the featured image size in second parameter based on this document

// without parameter -> Post Thumbnail (as set by theme using set_post_thumbnail_size())
get_the_post_thumbnail( $post_id );                   

get_the_post_thumbnail( $post_id, 'thumbnail' );      // Thumbnail (Note: different to Post Thumbnail)
get_the_post_thumbnail( $post_id, 'medium' );         // Medium resolution
get_the_post_thumbnail( $post_id, 'large' );          // Large resolution
get_the_post_thumbnail( $post_id, 'full' );           // Original resolution

get_the_post_thumbnail( $post_id, array( 100, 100) ); // Other resolutions

actually php cant check the responsive of screen. But if you want check the user run in mobile or not you can use this function.

if ( wp_is_mobile() ) {
    echo get_the_post_thumbnail( $post_id, 'large' ); 
} else {
    echo get_the_post_thumbnail( $post_id, 'thumbnail' ); 
}