Radial symmetry is the property of being shaped the same way in all directions. Any equal-sided polygon is radially symmetrical in a limited number of directions. The ultimate in radial symmetry is a circle. The closer to round, the more efficiently a given volume or area can be enclosed (least amount of surface material/volume.) In addition, rounder shapes have better Storm Resistance
A good compromise in terms of modularity, efficient use of materials, and scalability is the hexagon. Hexagons can be fit together without spaces, are a fairly wind- and wave-resistant shape in themselves, and can be combined into rounded shapes that maintain the characteristic. A good natural example of a strong structure is the honeycomb, an excellent combination of efficient use of materials, space, and self-reinforcing additive construction. Building individual hexagons and combining them after the fact does not achieve the ultimate in materials efficiency, but it does provide great structural strength.
The octagon is another shape that provides good storm resistance, and scalable combinations. However, octagons cannot be combined without interstices. This may be a desirable or undesirable aspect depending on design. Octagons are more nearly round, so more efficiently enclose space, but are not quite as strong in combination as hexagons. The interstices mean there is more material used in designs that combine octagons than for those that combine hexagons.