I'd say one page is too much, especially since they'll be pouring over your resume following the cover letter. Few paragraphs, write about how your particular skills are aligned with the job that you're applying for, how some of your prior experiences helped prepare you for such a position, etc, but be concise and direct.
And if possible address the cover letter to a specific person, if you can find out who's going to read it then you can make the letter even more personal.
I'm sure someone will disagree, but when I'm staring at a pile of 100 resumes, the cover page doesn't even get a glance. Straight to the pertinent info in the resume & on the next one.