The intercuspal width of an occlusal amalgam cavity preparation should be approximately 1/3 of the buccolingual width of the tooth. This ensures enough bulk of amalgam to withstand occlusal forces while preserving sound tooth structure. A preparation wider than 1/3 risks weakening the tooth and compromising resistance form, whereas a narrower width might not provide adequate access for complete caries removal and amalgam placement. Maintaining this dimension optimizes structural integrity and longevity of the restoration.

Reference: Sturdevant's Art and Science of Operative Dentistry, Roberson, 7th Edition.