Take a look at this plugin which allows that and much more
http://wordpress.org/extend/plugins/excerpt-editor/
Thanks for that – but though that plugin *does* do a billion things, it apparently doesn’t do the specific thing I want it to.
I had the same complaint! I just found a solution below, but note that you will probably have to repeat the fix after every WordPress version update:
http://www.optiniche.com/blog/13/change-the-excerpt-box-size-in-the-wordpress-admin-panel/
Here is the text from that page in case it disappears:
“If you regularly use your WordPress excerpt box, you may find its size a bit restrictive. You can alter it by altering your admin CSS file. It’s a quick fix and I’m here to walk you through it.
*Anywhere you see [WP-INSTALL], it means your WordPress installation directory.
Open the [WP-INSTALL]/wp-admin/wp-admin.css file in a text or HTML editor (make sure to back up first) and use your Find function to find #excerpt (Line ~361).
Change the height: 1.8em; to whatever height you are comfortable with. Knowing my own personal writing style, I changed it to height: 5em;
Save the file and upload it to your [WP-INSTALL]/wp-admin/ folder and you’re done. Login to your WordPress admin panel and click on Write to see the changes.”
Oh I just noticed you were asking about the excerpt field in the “dashboard area”. I was talking about the excerpt field in an edit window, or new entry window.