Jon Galloway

WordPress feeds offer a choice between Full Text or Summary. The Full Text option is preferable in a lot of cases (e.g. for a podcast), but the feed can grow very large.
One problem with a large feed is that if can exceed the <a href="http://support.google.com/feedburner/bin/answer.py?hl=en&answer=79009" rel="nofollow ugc">Feedburner 512K limit</a>. Your feed may be valid when you first set it up, but eventually when your feed gets big enough, Feedburner will stop updating.
This plugin gives you another setting (available on your Settings / Reading admin page) which sets how many posts should have Full Text. All posts after that will be shown as Summary. This gives you the best of both options – Full Text for new posts, and Summary for older posts. 
<h3>Readme Generator</h3>
This Readme file was generated using <a href='http://sudarmuthu.com/wordpress/wp-readme' rel="nofollow ugc">wp-readme</a>, which generates readme files for WordPress Plugins.

Excerpt Old Posts

This plugin solved my problem with feedbruner.



Greate idea

Shows excerpted content for older posts. This allows more items in the feed while working around the Feedburner 512K feed limit.

Md Abul Bashar

Just use Feedburner Email Subscriber service on your website sitebar widget.
Full <a href="http://www.codingbank.com/item/wp-feedburner-email-subscriber-wordpress-plugin/" rel="nofollow ugc">Documentations</a>
You can watch this videe 
https://www.youtube.com/watch?v=lqQnR_21UvQ

WP Feedburner Email Subscriber

Does the job

Just use Feedburner Email Subscriber service on your website sitebar widget..

Excerpt Old Posts vs WP Feedburner Email Subscriber