The dynamics of the alpha-relaxation in homogeneous mixtures of polystyrene (PS) and poly(cyclohexyl acrylate-stat-butyl methacrylate) (P(CHA-stat-BMA)) is analyzed within the framework of a concentration fluctuation model. The shape and widths of the dielectric relaxation spectra as well as the range of calorimetric glass transition in the mixtures are respectively associated with distributions of relaxation time and glass transition temperature arising from the presence of concentration fluctuations. The magnitude of these fluctuations is obtained by fitting the dielectric loss curves to a model function obtained as a convolution of a relaxation time distribution for the mixture with a function describing the dielectric loss of the dielectrically active component. The relaxation time distribution is calculated from William-Landel-Ferry (WLF) or Vogel-Fulcher-Tammann (VFT) free volume scaling with the assumption of a Gaussian concentration distribution in the samples. Fits of the measured cr-relaxation spectra which are dominated by P(CHA-stat-BMA) offer as a result the mean square concentration fluctuations [(delta phi)(2)] in the mixtures. The fluctuations are interpreted in terms of the random phase approximation, and it is shown that [(delta phi)(2)] is correlated to the length scale of cooperativity governing the relaxation process near the glass transition. The temperature and composition dependence of both [(delta phi)(2)] and the size of the domains of cooperativity in PS/P(CHA-stat-BMA) are determined.