With the v7.3 release the size adjustment for the description textarea has been moved from a specific configuration in the admin tool to the css. The default.css file contains the following to control that:

/*
 * Allow only vertical resizing of textareas.
 */

textarea {
	resize: vertical;
	/* Also make sure size does not go past the width for small-screens */
	max-width: 98%;
}

That allows the wysiwyg box to be stretched as needed. You can however make that taller by fixing the height like so but still allow it to resize vertically:

/*
 * Allow only vertical resizing of textareas.
 */

textarea {
	resize: vertical;
	/* Also make sure size does not go past the width for small-screens */
	max-width: 98%;
        
        height: 10em;

}

You will need to move that css from your default.css to your custom.css file to make that change. To make such a change or any change to the css to customize to your needs look to the edit custom.css file support wiki page

By default if you have the wysiwyg on for the client side within the admin tool the wysiwyg will default to be on at least for the first time the client places a listing. If they switch to remove the wysiwyg from use the browser will remember their choice and the wysiwyg will be off the next time they place a listing. But there may be times you want the wysiwyg to be on by default no matter their previous choice or always default off no matter their previous choice. You can easily make either of these happen with a little extra javascript with the page template used within the listing details collection page (the default design assigns the /main_page/cart_page.tpl template). To have the default to always make the wysiwyg ON no matter their previous choice add the following to the page template used on that page:

{add_footer_html}
<script>
 localStorage.setItem('tinyMCE','on');
</script>
{/add_footer_html}

To make the wysiwyg to always be off no matter their previous choice insert the following into that template:

{add_footer_html}
<script>
 localStorage.setItem('tinyMCE','off');
</script>
{/add_footer_html}

Most likely in the html head of that template.