The addition of metal particles, for instance, silver or copper, into polymer matrices is a relevant strategy producing novel antimicrobial materials. By using two particles with diameters around 10 nm and 45 µm, and polymers with different characteristics, the effect of filler size and matrix on the biocide behavior of polymer/copper composites was studied. The composites were prepared by melt mixing, and the ion release from these materials was used to obtain critical information about the processes involved. Regarding the effect of the particle size, our results for polypropylene showed that this variable drastically changes the release of copper from the matrix. The ion release rate from nanocomposites increased quickly exhibiting a sharp maximum during the first day; meanwhile, in microcomposites, the release rate increased slowly releasing lower ions. The relevance of particle size was confirmed by the antibacterial behavior of the samples as polypropylene with nanoparticles displayed larger activities against Staphylococcus aureus and Pseudomonas aeruginosa bacteria than microcomposites. These results further showed the relationship between copper ion release and antimicrobial behavior in polymer/metal composites. Our findings further revealed that the ion release from polymer composites could be improved by either increasing the hydrophilic characteristic of the matrix or by lowering its crystallinity. These observations allowed the conclusion that both the polymeric matrix and the size of the metal filler are relevant variables toward the design of antimicrobial composite materials.