Include a few examples of how you can contribute to the company's success. Writing the cover letter is not a test of what you've learned about the company. Don't feel obligated to include everything you've learned. There are many ways to write a good letter. Draft and revise yours until you're comfortable with it.
Few people have the time to read long letters, unless they're from friends.
In the last paragraph, tell the company what your next step will be. Include your phone number and ask the recipient to call if he or she has questions. Companies will rarely call immediately, so explain in your letter that you will call to check on your application's status and arrange an interview. Give the recipient a week to review your application, but make sure you're getting the attention you want. Always ask to arrange an interview.