in the gallery folder, there is a sub folder for each gallery. each sub folder contains an index.php page that generates the gallery.
in the index.php page, there is a line similar to:
$gallery = get_gallery(get_mediarss_url('gallery=' . $pg_id . '&thumb=62c&full=782p'));
this sets the image sizes to use with the following two arguments:
You may want to create custom image sizes to use in your gallery, see this post for details on how to create custom image sizes: