The available energy density supplied by conventional light sources prohibits the use of time-resolved volumetric Particle Image Velocimetry (PIV) with submillimetre helium-filled soap bubbles (HFSB) on a cubic metre scale. In connection with attempts to expand the maximum attainable measurement domain of PIV, a research project is carried out into the upscale of the seeding particles, capable of scattering more light. The production flexibility and scalability of HFSB is investigated by designing and improving the conventional HFSB nozzle generator used by Delft University of Technology. Experiments are performed on various nozzle designs, focusing on the average bubble diameter. Through geometrical scaling and further developments, the bubble diameter is scaled from 0.5 mm, produced by conventional nozzles, to 2.5 mm. The obtained upscale in bubble diameter corresponds to a 3-fold increase along all dimensions compared to the current operational domain for HFSB experiments. Furthermore, a novel physics-based model is proposed relating particle size to the nozzle orifice and flow rates of air and helium and is benchmarked with the acquired experimental data.