When particulate solids flow through an opening, the effective dimensions of the opening are smaller than the actual dimensions by an amount that is proportional to particle size. This applies to both unfluidized and fluidized flows. The theoretical basis is developed and a conceptual design outlined for an instrument that exploits the particle size correction to opening dimensions as a method for measuring or monitoring particle size. Particles are caused to flow sequentially through two fluidized beds, the second of which has a narrow slot in its side. It is demonstrated, using apparatus configured to simulate this instrument, that changes in particle size can be readily detected and continuously monitored. A good estimate of mean size can also be obtained, and it is expected that with appropriate calibration, this would be satisfactory for many process applications.