The influence of impeller diameter on crystal growth kinetics of borax decahydrate in a batch-cooling crystallizer of non-standard aspect ratio was evaluated. The dual-impeller configuration consisted of a pitched-blade turbine which was mounted below a straight-blade turbine on a single shaft. Three different impeller-to-tank diameter ratios were investigated. In all experiments, mixing was conducted at just-suspended impeller speed. To examine hydrodynamic conditions, mixing times were measured. The fluid flow pattern and velocity distribution were determined by computational fluid dynamics. Results showed that the smallest but also more regularly shaped crystals were produced in a system with standard diameter impellers. Product yield and power consumption were highest in this case.