We use scaling analyses to study the adsorption of A-B diblock copolymers on spherical colloidal particles of finite sizes in a selective solvent good for the B-block and poor for the A-block, while taking account of the competing process of self-association (micellization) of the copolymers. We show that for colloidal particles of a given size, the competition can be controlled by changing the lengths of the two respective blocks of the copolymer. Similarly, for a given diblock copolymer, the competition can be controlled by changing the size of the colloidal particles. In addition, we find that under conditions where adsorption is more favorable than micellization, a given diblock copolymer adsorbs preferentially to particles with a certain size. The latter effect can have potential applications for separation of particles of a given size from an initially polydispersed sample.