It MUST be just 1 page. It should be a very short, to the point intro and explanation of yourself. Details which are not essential or absolutely pertinent should not be included in the cover letter, they should go in your actual resume.
Effectively imagine a boss having 50 CV’s in front of him.
Will he read every single one? No.
Will he look at the cover letter of each first? Yes.
Will he read every detail of every cover letter? No
Will he just look for the main good points in a cover letter to decide whether he reads the actual CV? Yes
So should that cover letter be pages long? Absolutely not!
If your cover letter is an essay I guarantee you it will not get read. See the cover letter like an advertisement for a film – put all your good bits right there in front of the guy who’s job it is to trawl the CV’s and give him a short teaser to make him want to turn over to read your actual CV.
Using that concept, cover letter should have max 3-4 paragraphs with about 5 lines max each.
And remember to make it well written, short, snappy, very much to the point, include only your very exceptionally good bits that you think will get them interested in you as a candidate and ensure your last paragraph really gets across the reason you want that job.