Custom thumbnail sizes

You can use the built in filters such as

add_filter('maxgalleria_thumb_size_landscape_one_column', 'thumb_size_landscape_one_column', 10, 1);

function thumb_size_landscape_one_column($thumb_size) {
 return  array('width' => '800', 'height' => '506'));
}

Add these to your theme’s functions.php. You can find all the hooks used in get_thumb_size in http://maxgalleria.com/documentation/maxgalleria/hooks/. There are 30 hooks available to use.