Keep it to one page, and ideally under 400 words. Three to four short paragraphs is the standard structure: an opening line saying what role you’re applying for, a paragraph connecting your background to the role, a paragraph showing genuine interest in the organization, and a short closing line.
A cover letter that’s too long signals you haven’t edited it — recruiters are reading dozens of these, and brevity reads as respect for their time.
Quick answer: One page, under 400 words, three to four short paragraphs.