Microstructured membrane-assisted fluidized bed reactors have been proposed for hydrogen production as an efficient solution to strongly reduce bed-to-membrane mass transfer limitations (concentration polarization) adversely affecting other types of membrane reactors while maximizing the volumetric production capacity. In a previous work it has been demonstrated that a microstructured membrane-assisted fluidized bed can be successfully operated with state-of-the-art membranes in the turbulent fluidization regime with significantly reduced formation of densified zones. The effect of the particle size and the reactor size on the hydrodynamics of these novel reactors is not well understood. In this paper, the fluidization behavior of beds with different reactor sizes employing different particle sizes has been investigated using particle image velocimetry coupled with an advanced digital image analysis. It has been found that a bed with small width can be operated in the turbulent regime already at relatively low fluidization velocities. Also for reactors with a small bed width the extraction of gas via membranes results in the formation of densified zones close to the membrane walls, while the addition of gas results in an inversion of the solids circulation pattern. However, compared with bigger reactors, the influence of gas permeation via the membranes on the solids circulation patterns as well as bubble size and its distribution is much less pronounced in reactors with a smaller bed width. Also the effect of particle size on the hydrodynamics of the bed when extracting/adding gas through the membrane walls is discussed in this paper. The findings of this work give guidelines for tuning the fluidization regime, reactor size, and particle diameter, for a proper reactor design of microstructured membrane-assisted fluidized bed reactors that can offer significant advantages, viz, maximum volumetric production capacity while minimizing expensive reactant bypassing at the outlet and maximizing the product recovery.