How do you make larger images in the image attachment pages?

It'll be easiest if I walk you through it...

Take a look at this page: http://www.jasoncross.org/2009/09/11...-new-istuff/3/

Click any of the in-line images in that post, and it will take you to an attachment page with that image on it. The image on that attachment page is medium size (as seen here http://www.jasoncross.org/2009/09/11...istuff/genius/). If you click on THAT, you get taken directly to the .jpg.

What I want to do is make the image on that attachment page larger, so it will fill the width of my center column.

I know that when I insert the image into my post I can link to the image directly, but then I have no title or sidebars or anything...it's a direct link to the .jpg. I don't want that. I want the attachment page, I just want bigger pictures on there.

Other themes I have used had an Image Attachment Template file (image.php) that I could edit. I'd find where it says "<?php echo wp_get_attachment_image( $post->ID, 'medium' )" and change the medium to large. Then I can adjust the width of the large image in the regular wordpress Media settings.

My problem is I can't find the place in Atahualpa where it describes the size of image attachments. Can someone help me out here?