function remove_default_image_sizes($sizes) {
    unset($sizes['medium']);
    unset($sizes['large']);
    return $sizes;
}
add_filter('intermediate_image_sizes_advanced','remove_default_image_sizes');

global $_wp_additional_image_sizes;
foreach(get_intermediate_image_sizes() as $_size){
    echo $_size;
}

thumbnail
medium
medium_large
large