Employers across the board say cover letters should be short and succinct. Hiring managers have to scan hundreds and sometimes even thousands of resumes and cover letters, so writing one that's more than a page long can easily work against you [source: Ludwick]. The general rule is to keep them around 200 to 250 words. And what you write should be a concise summary of what you can do for the company and why a recruiter should move on and look at your resume [source: Sundar].
Another way to look at it -- limit yourself to four paragraphs: an introduction; brief summary of your applicable skills and achievements; some mention of the company that's hiring to show you've done some research and know a little bit about it; and then a simple sign-off [source: Learn Vest]. And definitely avoid the clichéd opener: "Enclosed please find my resume highlighting my experience and skills ..." [source: Sundar].