Edited according to first comments 和Pastebin代码：

<?php /* You can also leave \'action\' blank: action="" */ ?>
<form method="post" action="<?php echo $_SERVER[\'PHP_SELF\']; ?>">
<select name="my_size" id="size" class="postform" onchange="submit();">
        <option selected="selected">Choose a size</option>
        <option value="10">10</option>
        <option value="20">20</option>
</select>
</form>
<?php /* Reset filter */ ?>
<p><a href="<?php the_permalink(); ?>">Clear filter</a></p>

<?php
if( !isset($_POST[\'my_size\']) || \'\' == $_POST[\'my_size\']) {

    // unfiltered product list here

}
else {

    // Get dropdown value
    // Don\'t forget to sanitize this!
    $size = $_POST[\'my_size\'];

    // Create new query
    $query = new WP_Query( array(
        \'post_type\'=> \'product\', // your CPT
        \'post_status\' => \'publish\',
        \'meta_key\' => \'size\',
        \'meta_value\' => $size, // Dropdown value
    ) );

    // Loop
    if($query->have_posts()):
        while( $query->have_posts() ): $query->the_post();
            // Product content
        endwhile;
    endif;

    // reset query to default
    wp_reset_postdata();

}