The room temperature synthesis of ZIF-8 micro- and nano-particles was investigated using a mixed methanol-water solvent system. ZIF-8 particles of good quality and high crystallinity were obtained. Response surface methodology was used to determine the effect of the synthesis conditions on the ZIF-8 yield, particle size distribution, and mean particle size. The ligand/metal salt molar ratio followed by the amount of sodium formate (the deprotonating agent) and then the amount of water (i.e., the composition of the mixed solvent) respectively had the largest effects on both the ZIF-8 yield and particle size. Results showed that mixing of solvents with different strengths in producing ZIF-8 crystals is a practical method to size-controlled synthesis of ZIF-8 particles. This method is more favorable for industrial-scale ZIF-8 synthesis than using excess amounts of ligands or chemical additives (like sodium formate). In addition, ZIF-8 samples with different mean particle sizes (100, 500, and 1000 nm) were used for CO adsorption and the mid-sized ZIF-8 particles had the highest adsorption capacity.