How to set the default image size in WordPress galleries From http://www.wprecipes.com/how-to-set-the-default-image-size-in-wordpress-galleries The desired size can be changed on line 5.

new_gist_file_0

remove_shortcode('gallery');
add_shortcode('gallery', 'custom_size_gallery');
 
function custom_size_gallery($attr) {
     $attr['size'] = 'medium';
     return gallery_shortcode($attr);
}