An estimate method that used a parameter written Co is proposed in this paper. It's possible to estimate the damping performance by using the parameter Co between different size structures. Because the parameter Co is dimensionless, the size of the structure is irrelevant to estimate. The applicability of this method was proven to be high by using the observed data. In addition, this paper proposes a method to presume the parameter of the proportional damping equation considering the scale effect. And, the characteristics of the proposed equation and the relationship with Co were identified.