Biopolymer nanoparticles can be formed by heating globular protein–ionic polysaccharide electrostatic complexes above the thermal denaturation temperature of the protein. This study examined how the size and concentration of biopolymer particles formed by heating β-lactoglobulin–pectin complexes could be manipulated by controlling preparation conditions: pH, ionic strength, protein concentration, holding time, and holding temperature. Biopolymer particle size and concentration increased with increasing holding time (0 to 30 min), decreasing holding temperature (90 to 70 °C), increasing protein concentration (0 to 2 wt/wt%), increasing pH (4.5 to 5), and increasing salt concentration (0 to 50 mol/kg). The influence of these factors on biopolymer particle size was attributed to their impact on protein–polysaccharide interactions, and on the kinetics of nucleation and particle growth. The knowledge gained from this study will facilitate the rational design of biopolymer particles with specific physicochemical and functional attributes.