There is no way to do this as far as I have found using the_excerpt().
There is a similar StackOverflow question here.
The only thing I have found to do is write a new function to take the_excerpt()'s place. Put some variation of the code below into functions.php and call limit_content($yourLength) instead of the_excerpt().
function limit_content($content_length = 250, $allowtags = true, $allowedtags = '') {
global $post;
$content = $post->post_content;
$content = apply_filters('the_content', $content);
if (!$allowtags){
$allowedtags .= '<style>';
$content = strip_tags($content, $allowedtags);
}
$wordarray = explode(' ', $content, $content_length + 1);
if(count($wordarray) > $content_length) {
array_pop($wordarray);
array_push($wordarray, '...');
$content = implode(' ', $wordarray);
$content .= "</p>";
}
echo $content;
}
(Function credit: fusedthought.com)
There are also "advanced excerpt" plugins that provide functionality like this you can check into.