>SSTO because with increased size, the weight of the propellant rises as the
>cube function while the structural weight for tanks, etc, rises roughly as
> the square function.

This is a common misconception.

Propellant tanks, even for pump fed rockets, are dominated by internal pressure forces. The tank mass scales with the volume, not the surface area. For a given pressure, a tank that doubles in width, height, and length must also double in wall thickness, exactly tracking the contained volume.

Several things do get easier with larger size, but there is nothing as powerful as a square/cube law going for it.

John Carmack
www.armadilloaerospace.com