Struvite crystallization has been widely investigated, but little attention has been focused on particle size, an important characteristic in fertilizers that affects the agronomic response. Although there is no standard, it is an interesting parameter to be controlled as each crop and soil require a specific fertilizer size. Few attempts have been made to increase particle size. In air-lift reactors, the up-flow velocity could be used as a controlling parameter for particle size. It is hypothesized that the up-flow velocity determines the minimum theoretical equivalent diameter that can be recovered. Its effectiveness as a controlling parameter for particle size was evaluated in a combined air-lift reactor at different up-flow velocities (from 13.3 to 26.3 m h−1). Particle size was found to be directly correlated with the up-flow velocity, and according to the theoretical approach about the minimum equivalent diameter. Moreover, particle growth was promoted by operating the crystallizer in continuous mode at the best up-flow velocity (22.6 m h−1), having an increase of the mean particle size in time (138.6 h) from 201 μm to 314 μm, and recovering particles up to 800–1000 μm. The up-flow velocity control did not affect the phosphate recovery rates (>90%) and XRD confirmed that only pure struvite crystals were recovered. Therefore, this study shows that struvite can be recovered from a waste stream and control its particle size using the up-flow velocity to adjust to the customers’ requirements.