Droplet microfluidics provides a controlled route for producing polymeric microparticles, but the relationship between droplet hydrodynamics and final particle size remains insufficiently understood. In this study, computational fluid dynamics (CFD) analyses were performed to investigate how circulation within droplets influences PLGA microparticle formation in a 100 μm flow-focusing microfluidic device. A 2D CFD model was developed and validated against μ-PIV velocity measurements and experimentally measured droplet sizes, giving good agreement with the observed flow profiles and droplet formation behaviour. The effects of flow rate ratio and total flow rate were analysed using swirling strength and circulation time as hydrodynamic descriptors of internal recirculation. Results showed that increasing the flow rate ratio reduced droplet size by approximately 39% and enhanced internal circulation, resulting in a 64% reduction in particles' size. At a constant flow rate ratio, increasing the total flow rate produced only slight changes in droplet size but significantly increased swirling strength and reduced circulation time, leading to a 35.3% decrease in particle size. These findings indicate that particle size is affected not only by droplet size but also by the internal hydrodynamic conditions. The work provides a hydrodynamic framework for polymeric microparticle formation, enabling the prediction of particle size in droplet-microfluidic systems.