Good cover letters should take ¾ to 1 page in length maximum. Anything longer than that will make the recruiter lose focus, potentially resulting in missing the most important aspects you want to stress.
Always keep in mind that your goal is to spark interest in the recruiter so that they see and understand the added value you will bring to the job. Adding interests on your resume is commonly met, but when it comes to your cover letter, they should definitely be left out.