When you add a cover letter, you should really keep it to less than one page. The cover letter is basically a teaser to intrigue the person who is screening your application.
What you want to do is basically give three top reasons why you're a good candidate for this job. Then give a piece of evidence to back it up. Each of these reasons needs maybe a couple of sentences, then you add an introduction, why you are excited about the firm and how you heard about this position, and then you add a closing statement regarding to your enthusiasm for the job.
Basically, this entire thing is to give them a taste that you have the skills that they need, and to make them interested enough to have them call you in for an interview. If you tell your whole life story it might be too much.