A cover letter should almost always fit on a single page, and ideally within three to four short paragraphs. Longer letters are rarely read in full, and length alone does not make a stronger case.
Tone should be professional but genuinely conversational — avoid overly formal or stiff language, but also avoid excessive casualness. Read your letter aloud; if it sounds like something you would actually say, the tone is likely right.
Formatting should mirror your CV: same font, consistent header style, and a clear structure with proper greeting and closing. Address a specific person or team when possible rather than a generic "To Whom It May Concern."
Finally, always proofread carefully. A cover letter with typos undermines an otherwise strong application, since it is one of the few places where attention to detail is directly visible to the reader.