emoop

add_action( 'woocommerce_after_shop_loop_item_title','display_sizes' );
function display_sizes(){
  global $product;
  
  $variations=$product->get_available_variations();
  $count=sizeof($variations);
  $counter=0;
  echo '<span class="sizes">Sizes:';//sizes or whatever u want
  foreach($variations as  $var){  
    foreach($var['attributes'] as $name=>$val){

emoop

<?php
 /* if you have get a custom field  use this filter*/
 
 add_filter('woocommerce_api_product_response','get_custom_field');
 
 // '_brand' is my custom field
function get_custom_field($product){
 $product['brand']=get_post_meta($product['id'],'_brand',true);
 return $product;
}