Had the same problem, here is a solution that works in Woocommerce 2.4. You can add this to your functions.php file. Although I use Code Snippets to make it easier to manage things.
/* Change Woocommerce Default Value Text */
add_filter('gettext', 'choose_option');
add_filter('ngettext', 'choose_option');
function choose_option($translated) {
$translated = str_ireplace('Choose an option', 'Select Size', $translated);
return $translated;
}
This will change the option across your entire site.