В контроллере catalog\controller\product\compare.php заменить код в строках 126-129
'weight' => $this->weight->format($product_info['weight'], $product_info['weight_class_id']),
'length' => $this->length->format($product_info['length'], $product_info['length_class_id']),
'width' => $this->length->format($product_info['width'], $product_info['length_class_id']),
'height' => $this->length->format($product_info['height'], $product_info['length_class_id']),
на
'weight' => ($product_info['weight'] > 0) ? $this->weight->format($product_info['weight'], $product_info['weight_class_id']) : '',
'length' => ($product_info['length'] > 0) ? $this->length->format($product_info['length'], $product_info['length_class_id']) : '',
'width' => ($product_info['width'] > 0) ? $this->length->format($product_info['width'], $product_info['length_class_id']) : '',
'height' => ($product_info['height'] > 0) ? $this->length->format($product_info['height'], $product_info['length_class_id']) : '',
А в представлении catalog\view\theme\default\template\product\compare.tpl заменить в строке 106
<td><?php echo $product['length']; ?> x <?php echo $product['width']; ?> x <?php echo $product['height']; ?></td>
на блок
<?php if ($product['length'] && $product['width'] && $product['height']) { ?>
<td><?php echo $product['length']; ?> x <?php echo $product['width']; ?> x <?php echo $product['height']; ?></td>
<?php } else { ?>
<td></td>
<?php } ?>