Wordpress's excerpts kinda suck to customize. This function allows you to generate an excerpt at a desired word count with an optional "Read More" link. It also trims the result back to the last complete sentence to keep things pretty.

gistfile1.php

function custom_excerpt($limit = 55, $more = false){
	
	// from wp_trim_excerpt() in /wp-includes/formatting.php
	$text = get_the_content('');
	$text = strip_shortcodes( $text );
	$text = apply_filters('the_content', $text);
	$text = str_replace(']]>', ']]>', $text);

	// wp already has a trim function 
	$text = wp_trim_words($text, $limit, ''); // pass an empty string in as $more, so we can define our own
	
	// let's trim this back to the last sentence
	$punctuation_locations = array(
		strrpos($text, '.', -1),
		strrpos($text, '?', -1),
		strrpos($text, '!', -1),
		strrpos($text, ')', -1),
		strrpos($text, '"', -1)
	);

	$pos = max($punctuation_locations);
	       
    	if($pos != false) {
        	$text = substr($text, 0, $pos+1);
   	}

	// tack on the read more if asked to
	if( $more ){
		$text .= ' <a href="'.get_permalink().'" title="'.get_the_title().'">Read more...</a>';
	}

	return wpautop($text);
}