The influence of colloid diameter on the surface plasmon resonance (SPR) response of a colloidal Au modified Au film is described. Adsorption of 30-59 nm diameter colloidal Au on the surface of a 47 nm thick Au film results in dramatic, size-dependent shifts in plasmon angle, minimum reflectance, and curve breadth. Each of these parameters is observed to increase with particle diameter and particle coverage, eventually reaching a plateau when the attenuation of the SPR curve is maximized. The angle of maximum change in reflectance is observed to be independent of particle size; thus, the reflectance at that angle is proposed as a method for quantifying the perturbation induced by a particular size and coverage of colloidal Au. These results are discussed in terms of the detection limits and dynamic range of colloidal Au-amplified SPR biosensing.