It’s always a good idea to include a cover letter for any job, but a cover letter for an academic job can be particularly important. Academic application materials require that you provide information about your technical skills and about who you are as a person, because a teaching job is so heavily based on how you interact with students, and a letter is a great vehicle for communicating both. If you’re not well-versed in using cover letters, use the cover letter builder from ResumeHelp to help you create a cover letter.
You should aim for anywhere between half a page and 3/4 of a page. Any longer than a single page and you risk losing the interest of the reader. Any shorter than half a page and it may look like you’re seriously underqualified.
Remember that experience doesn’t have to just be about previous teaching positions. If you’re a graduate student, you can talk about educational experiences you’ve had, for example. Just highlight the experience that you do have that contains skills critical for the job, regardless of what type of experience it is.