How to check whether a post exceeds 300 words

basically you need to change the default EXCERPT length of your posts.

there are a couple of ways to solve this issue:

1. if you aren’t familiar with PHP

Install this plugin at : http://wordpress.org/plugins/advanced-excerpt/

This will allow you to change the the excerpt length from the admin panel.

1. if you are familiar with basic PHP

Go to your functions.php in your theme at /wp-content/themes/YOUR_THEME_NAME/functions.php

Add the following code:

function different_excerpt_length($length) {
    return 200;
}
add_filter('excerpt_length', 'different_excerpt_length');

You can change 200 to any number you want like 300.

As for showing your summary instead of the full post, you can follow the instructions from wordpress.com like so : http://en.support.wordpress.com/splitting-content/more-tag/

Hope it helps!