Change text for dropdown of variable products to choose – WooCommerce

Edit red coloured for your situation and add this code to functions.php

function my_dropdown_variation_attribute_options_html($html, $args){
$html = str_replace('Choose an option', 'Size options', $html);
return $html;
}
add_filter('woocommerce_dropdown_variation_attribute_options_html', 'my_dropdown_variation_attribute_options_html', 10, 2);

Change text for dropdown of variable products to choose – WooCommerce