Grievance letters, or claim letters, should have the same margin size as any business letter: 1 inch, all around. Business-sized margins lend a claim letter an aura of professionalism that makes it harder to ignore.
Grievance letters are essentially formal complaints and should be structured formally in all respects. Align all text to the left, write a full heading including the address of the person or company directly at fault, begin with a salutation of "Dear Mr./Mrs." and end with a formal signature. Make sure to be clear and concise, using exact dates and times and keeping as calm a tone as possible.