My advice is to analyze all of the documents the institution is requesting and then use your cover letter to address any strengths/details that won't be covered elsewhere. Does the college/university require a statement of purpose or academic philosophy? Those documents ask for your views on stage management as an academic discipline. If the school does not ask for such a statement, see if you can work it into your cover letter. Another good use of a cover letter is to explain why the position interests you so much. Show your personality and style since it may be difficult to separate yourself from the other candidates in the more formal documents of your application. I concluded a search last spring and the majority of cover letters ran a full page to to just short of two full pages.