In terms of capacity, the tiniest deck boxes are around 20 gallons, often shaped like a cube. Medium deck boxes in the 90- to 120-gallon range are perhaps the most popular, which is where our best overall falls. Note that not all deck boxes are built to double as extra seating. MaterialAlmost all outdoor storage and deck boxes are water-resistant to some degree. Wooden deck boxes typically look more upscale than resin, but they also require more maintenance and may need to be stored indoors for the winter.