We report on a novel microsynthesis method for easy controlling the size of silica micro- and nanoparticles. Colloids are synthesized in a microfluidic chamber by layer-wise adsorption of silica to precipitated nuclei. We prepare samples of spherical particles with different numbers of coatings and find that particles with less than 15 coating layers are uniform in size. Particles with more coatings are disperse. This finding is attributed to peptization of nanoparticles and independent growth of secondary nanoparticles. For all numbers of layers, the maximum particle size increases with the number of coatings.