Try do not exceed 200 words.
Your opening paragraph should mention how you found out about the position (magazine article, classified ad, contact through a placement office or business acquaintance, etc.) and should illustrate your knowledge of the company and its goals. Subsequent paragraphs should indicate your compatibility with the position; you should also mention your enclosed resume at some point, using it to demonstrate your qualifications.
samples : #1 , #2 , #3 , #4 , #5