The size distribution and the rheological properties of dispersions of biological colloids are relevant quality attributes for a variety of industrial applications, including pharmaceutical, food, and cosmetic products. For instance, the biophysical properties of monoclonal antibodies and therapeutic proteins, which represent an important class of drugs in the pharmaceutical market, are important for their safety and efficacy. In this work, we apply a microfluidic diffusion platform to analyze protein sizes and interactions in high-concentration antibody solutions directly in the liquid state with minimal perturbation of the sample. We show that this method provides size distributions in a size range scaling from a few angstroms to hundreds of nanometers. The detection sensitivity of the technique is independent of the particle size, and the method provides number-average distributions, enabling the simultaneous detection of both monomeric species and soluble aggregates. We further show that the same platform can be applied to measure viscosity-scaling effects in crowded environments by probing the Brownian motion of several tracers with different sizes. Such tracers experience a shift from the microviscosity to the macroviscosity of the sample at a critical probe size that is equal to the characteristic dimension of the main components of the dispersions. The technique simultaneously provides quantitative measurement of the microrheological properties and the macroviscosity of the sample, as well as information on the characteristic size of the components of the solution. Overall, these methods represent attractive tools in the context of the analysis of sizes and interactions of proteins in both diluted and high-concentration solutions during development, manufacturing, and formulation.