If you have to extend the backend product grid of Woocommerce by an custom attribute , you have to follow the steps below to add product attribute columns to Woocommerce backend grid:

1. Register and sort  the new columns via a filter hook (‘manage_edit-product_columns’):

   <?php 
   add_filter( 'manage_edit-product_columns', 'add_columns_to_product_grid', 10, 1 );
   
   const BACKEND_PRODUCT_GRID_FIELD_SORTORDER
   = [
       'cb',
       'thumb',
       'name',
       'pa_size_text',
       'sku',
       'is_in_stock',
       'price',
       'product_cat',
       'product_tag',
       'featured',
       'product_type',
       'date',
       'stats',
       'likes'
   ];
   
   /**
    * Registers new columns for the backend products grid of Woocommerce.
    * Additionally it sorts the fields after
    * self::BACKEND_PRODUCT_GRID_FIELD_SORTORDER. Fields not included in
    * self::BACKEND_PRODUCT_GRID_FIELD_SORTORDER will be attached to the end of
    * the array.
    *
    * @param array $aColumns - the current Woocommerce backend grid columns
    *
    * @return array - the extended backend grid columns array
    */
   public function add_columns_to_product_grid( $aColumns ) {
       $aColumns['pa_size_text'] = __( 'Unit size', 'sheldon_misc' );
       #unset($aColumns['thumb']);
       $aReturn = [];
       foreach ( self::BACKEND_PRODUCT_GRID_FIELD_SORTORDER as $sKey ) {
           if ( isset( $aColumns[ $sKey ] ) ) {
               $aReturn[ $sKey ] = $aColumns[ $sKey ];
           }
       }
   
       /**
        * search additional unknown fields and attache them to the end
        */
       foreach ( $aColumns as $sKey => $sField ) {
           if ( ! isset( $aReturn[ $sKey ] ) ) {
               $aReturn[ $sKey ] = $sField;
           }
       }
   
       return $aReturn;
   }

2. Add an action hook to fill in the respective value of each row (‘manage_product_posts_custom_column’):

   <?php
   add_action( 'manage_product_posts_custom_column', 'add_columns_value_to_product_grid', 10, 2 );
   
   /**
    * Adds the respective value of the custom attribute to each row of the
    * column
    *
    * @param string $sAttributeCode
    * @param int    $iPostId
    */
   public function add_columns_value_to_product_grid(
       $sAttributeCode,
       $iPostId
   ) {
       if ( $sAttributeCode == 'pa_size_text' ) {
           $oProduct  = new WC_Product( $iPostId );
           $sSizeText = $oProduct->get_attribute( 'pa_size_text' );
           echo esc_attr( $sSizeText );
       }
   }

For simplicity I only show the raw functions here. In general it is a really good Idea to encapsulate the code into a plugin class! 😉

Further information on how to add product attribute columns to Woocommerce backend grid

For this short article I was inspired by this Stack Overflow question:

https://stackoverflow.com/questions/23858236/how-to-add-remove-columns-in-woocommerce-admin-product-list