Thanks macropod,
Your solutions works. But not exactly as expected.
You quoted "Those sentences should not contain more than 500 characters."

But what I am expecting is each PARAGRAPH ( Not sentences) should not contain more than 500 characters.
One paragraph can itself contain N number of sentences. But total characters should not exceed 500.
The reason behind this is that, one PPTX slide with font size 25 can store maximum 500 characters.

My idea is to break a document in paragraphs. Each paragraph can contain many sentences, but all to gather they should contain max 500 characters.
I know you are very very near to the solution.
I am C# expert, I don't understand VB else I would have solved this.
Your help is highly appreciated.
Thank you very much.