feed – summary / full

I realized that Feedburner’s Summary Burner doesn’t use the number of characters we set. No matter what I do, the feed always has +- 200 characters, and this doesn’t happen only to me.

I’d like to know what does the “Summary” button on the Reading Subpanel on WP does. What is considered to be the summary here?
If I didn’t write an excerpt it will take the first 55 words? Can I change this number?

Thanks