BiFeO3 nanoparticles with controlled particle size were synthesized via a facile co-precipitation method. The samples were characterized by x-ray powder diffraction, transmission electron microscopy, atomic force microscopy and UV-vis diffuse reflectance spectroscopy. By controlling the timing of base addition during the co-precipitation process the particle size can be controlled easily. It was found that longer base addition times, i.e. slower dropping rates, resulted in smaller particle sizes exhibiting narrower band gaps. The smallest BiFeO3 particles prepared by this method are best described as circular flake-like nanoparticles with a diameter of 10-20 nm and a height of approximately 2.5-7 nm. To the best of our knowledge, these particles are the smallest BiFeO3 particles ever prepared by a co-precipitation method. Furthermore, our synthetic protocol allows the facile synthesis of size-tunable, pure-phase BiFeO3 nanoparticles with a low concentration of surface defects and local strain all leading to excellent efficiencies in photodegradation reactions of Rhodamine B in an aqueous solution under visible light irradiation. We show that the different particle sizes affected largely the resulting band gaps as well as the photocatalytic activity of the corresponding samples leading to narrower band gaps and improved photocatalytic activity in smaller particles.