How to cahange Thumbnail size for mobile browser

You can use this snippet https://wp-puzzle.com/docs/basic/thumnbail-size.html with if ( wp_is_mobile() ){ ... }

Like it:

function peach_set_thumbnail_size( $thumbnail_size ) {
    if ( wp_is_mobile() ){
	$thumbnail_size = 'full';
    }
    return $thumbnail_size;
}
add_filter( 'basic_singular_thumbnail_size', 'peach_set_thumbnail_size', 10 );