Mention the exact job opening and number (if applicable) in the first paragraph. The purpose of a cover letter is to introduce you and briefly summarize your professional background.On average, it should be around 250 to 400 words long. Always write a cover letter from scratch. An open cover letter, also known as an unsolicited or cold cover letter, is a document describing your interest in working for a company. Indicate the job for which you are applying.