Cube 6 – 6 × 6 × 6 in
Compact and lightweight (often below 250 cubic inches), great for tight, snug items like small electronics, candles, or jewelry. For perspective, it’s roughly the volume of a small hardcover book. 

Cube 7 – 7 × 7 × 7 in
Just a bit larger—excellent for soap gift sets, delicate decorations, or premium candles. Size-wise, imagine filling it with a standard paperback book.

Cube 8 – 8 × 8 × 8 in
A go-to mid-size cube box. Perfect for fragile items like glassware or small kitchen gadgets. FedEx includes this in its "flat-rate medium" range (≈251–650 cu in). 

Rectangle 9×6 – 9 × 6 × 6 in
Slim yet practical: ideal for shipping phone accessories, slim decor, or small hardback books.

Cube 9 – 9 × 9 × 9 in
Versatile for a variety of products—stackable, sturdy, and still manageable. Can hold multiple items like packaged cosmetics or small decor pieces.