Quick answer: use one opening sentence, one proof paragraph, and one close. Keep only the evidence that changes the hiring manager's view of your fit.
Use a 3-part structure
Short does not mean thin. Use one sentence for fit, one paragraph for proof, and one sentence for next step.
Cut repeated resume details
If a detail already appears clearly in your resume, keep it out unless it explains why this role is the right match.
Make the proof do more work
One strong result can replace three generic strengths. Ask AI to compress proof, not remove it.
Prompt
Create a cover letter under 200 words using one opening sentence, one proof paragraph, and one closing sentence. Keep only evidence that matches the job description.