Not more than one page. Concentrate. Write the important.
Address the responsible HR officer personally (not “Dear Sir or Madam”, but “Dear Mrs. Ernoult”. Just for example.)
Write one or two concise subject line(s). Not more. (You find further hints and examples in the documents linked below.
Write a concise intro too. HR officers don’t have time. Start with something that excites him/her. But do not try to be too “witty”. Avoid standardized phrases, but stay businesslike. Try to be authentic, but serious.