Risolto,
la dicitura corretta è:
if (function_exists( 'add_theme_support')) {
add_theme_support( 'post-thumbnails');
}
if ( function_exists('add_image_size')) {
add_image_size( 'full-size', 9999, 9999, false );
add_image_size( 'slider', 980, 9999, false );
add_image_size( 'portfolio-single', 550, 9999, false );
add_image_size( 'small-thumb', 50, 50, true );
add_image_size( 'grid-thumb', 230, 180, true );
}
Successivamente, ho installato il plugin Regenerate Thumbnails (wordpress . org/plugins/regenerate-thumbnails) e rigenerato le miniature.