The influence of shape, size, and internal structure of CO aerosol particles on mid-infrared spectra is modeled for aggregates in the size range between 1 and 100 nm. Combining the vibrational exciton model with a molecular dynamics approach, we identify spectral features that are characteristic for the shape of the particles and for their internal structure (crystalline, amorphous, and partially amorphous) over the whole particle size range. The characteristic size-dependent patterns in the spectra of small particles (<10 nm) are lost for particle ensembles with typical size distributions. The comparison with calculated infrared spectra allows us to analyze experimental spectra of large particles (10-100 nm). Furthermore, we extract refractive index data for crystalline, amorphous, and partially amorphous CO ices from calculated infrared particle spectra.