It’s very simple to change the text of default Magento choose an option to size or whatever we want.

Go to your theme file location

app/design/frontend/YOUR-THEME_NAME(default)/default/template/catalog/product/view/type/options/configurable.phtml

Rplace this code 
 <script type="text/javascript">
        var spConfig = new Product.Config(<?php echo $this->getJsonConfig() ?>);
 </script>

to this one

<script type="text/javascript">
    <?php
        $jsonConfig = $this->getJsonConfig();
        $jsonConfig = str_replace("Choose an Option...", "Size", $jsonConfig);
    ?>
    var spConfig = new Product.Config(<?php echo $jsonConfig; ?>);
</script>