More exactly, since Size goes by a factor of 4 for weight (and hence volume), the scaling factor for linear measures is the cube root of four, which is approximately 1.6. So, that 3' Medium broadsword becomes a five foot Big broadsword, a seven and a half foot Large broadsword, and, yes, a twelve-foot long Huge broadsword for a mountain giant that stands 18-20 feet tall.