Ceramic is a better material than metal to use in the corrosive environment or at an elevated temperature. But ceramic is very brittle. Therefore the combined use of ceramic and metal is needed. A simple and sufficiently strong joining technique between ceramic and metal is essential in that case. We thought that shrink fit is one of the most available techniques, compared with welding, diffusion bonding brazing. In the previous work, we made clear that the existence of surface roughness reduced the strength of the shrink fit experimentally and theoretically. But we didn't change the sizes of the specimens in that work. It is thought that the degrees of the reduction of the strength will vary with the size of the outer diameter of the shaft, although the surface roughnesses are the same. In this paper, the experiments were carried out to obtain the strengths of the shrink fits of various sizes of specimens. And we investigated how the strengths of the shrink fits were reduced by the relationship between the surface roughness and the sizes of the specimens, experimentally and theoretically.