How to set the default image size in WordPress galleries

Simply paste the following snippet into your functions.php file:

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

The desired size can be changed on line 5.

Thanks to emoticode for the cool tip!