Hook onto the edit-tags page and insert some additional styling to adjust the textarea, the ID remains the same for post tags, categories and i think other taxonomies to..

Adjust CSS for taxonomy management page

Example only, tailor to suit you needs.

add_action( 'admin_print_styles-edit-tags.php', 'resize_description_field' );
function resize_description_field() {
    ?>
    <style type="text/css">
        textarea#tag-description { height:300px!important; }
    </style>
    <?php
}

Hope that helps..