Textarea.php

<?php

/**
 * @file
 * Contains \Drupal\config_translation\FormElement\Textarea.
 */

namespace Drupal\config_translation\FormElement;

use Drupal\Core\Language\LanguageInterface;

/**
 * Defines the textarea element for the configuration translation interface.
 */
class Textarea extends FormElementBase {

  /**
   * {@inheritdoc}
   */
  public function getTranslationElement(LanguageInterface $translation_language, $source_config, $translation_config) {
    // Estimate a comfortable size of the input textarea.
    $rows_words = ceil(str_word_count($translation_config) / 5);
    $rows_newlines = substr_count($translation_config, "\n" ) + 1;
    $rows = max($rows_words, $rows_newlines);

    return array(
      '#type' => 'textarea',
      '#rows' => $rows,
    ) + parent::getTranslationElement($translation_language, $source_config, $translation_config);
  }

}