I got the answer to this from:
http://wordpress.org/support/topic/theme-function-set-default-image-size-for-gallery-shortcode-feature?replies=6
I basically changed ‘thumbnail’ to ‘full’ in the media.php file:
extract(shortcode_atts(array(
‘order’ => ‘ASC’,
‘orderby’ => ‘menu_order ID’,
‘id’ => $post->ID,
‘itemtag’ => ‘dl’,
‘icontag’ => ‘dt’,
‘captiontag’ => ‘dd’,
‘columns’ => 3,
‘size’ => ‘thumbnail,
‘include’ => ”,
‘exclude’ => ”
), $attr));
No! Never, ever, edit WordPress core scripts. And do not encourage others to do so. Editing core scripts can bring down your entire site and/or open security holes for hackers to use.
esmi is right, if you read to the bottom of that post you referenced it shows you how to handle that modification. In your theme’s function.php add the code
remove_shortcode('gallery', 'gallery_shortcode');
add_shortcode('gallery', 'my_gallery_shortcode');
Then create your custom function “my_gallery_shortcode” (or whatever you name it), copy the original code from the gallery_shortcode function in media.php and paste it into your custom function and then feel free to edit away until you get what you want. So your functions.php would look something like this.([…] means abbreviated, there is a bit of code in that funcion lol)
function my_gallery_shortcode($attr) {
[...]
extract(shortcode_atts(array(
'order' => 'ASC',
'orderby' => 'menu_order ID',
'id' => $post->ID,
'itemtag' => 'dl',
'icontag' => 'dt',
'captiontag' => 'dd',
'columns' => 1,
'size' => 'large',
'include' => '',
'exclude' => ''
), $attr));
[...]
}
remove_shortcode('gallery', 'gallery_shortcode');
add_shortcode('gallery', 'my_gallery_shortcode');
