You can manually insert the “more” tag anywhere in your post to limit the excerpt length.
To do this globablly, you will need to use a child theme. Add this to your child theme’s functions file:
function custom_excerpt_length( $length ) {
return 20;
}
add_filter( 'excerpt_length', 'custom_excerpt_length', 999 );
Change “20” to however many words you would like your excerpt to be.
Read this for more information:
http://codex.wordpress.org/Function_Reference/the_excerpt