Content/Excerpt length control for a specific loop?

Is it possible to control the length of the content or excerpt for a specific query/loop?

I have come across the following code, but this changes the length for all excerpts, I want it focused on a specific custom query.

    add_filter('excerpt_length', 'my_excerpt_length');
function my_excerpt_length($length) {
return 20; }

Try this: http://pippinspages.com/tutorials/better-wordpress-post-excerpt-revamped/