We propose to use the correlation dimension as a measure of surface roughness of protein molecules which varies with the size of guest molecules. Smaller guest molecules can probe the finer details of surface corrugation than the : larger ones. Hence the correlation dimension has the larger value for smaller guest molecules. Molecular dynamics simulations of a protein molecule at constant temperature and pressure show that the magnitude of structural fluctuation and the coefficient of self-diffusion vary with the size of solvent molecules and correlate nicely with the correlation dimension.