The paper presents an analysis of measurement results on cluster properties in a two-dimensional lab-scale cold fluidized bed operating in turbulent and circulating fluidized bed modes. Experiments were carried out with Geldart B particles of (Sauter mean) diameters varying from 0.256 mm to 0.440 mm. Different particle size distributions and bed mass were used along with different superficial gas velocities in the range of 2.75 m/s to 4.00 m/s. High-speed images of the bed were taken in a controlled lighting environment. Analysis of the experimental images was conducted to determine the horizontal length scale distribution of the solid clusters along the 3 m high CFB. Additionally, the solid hold-up inside the clusters was evaluated. The effects of particle size, bed mass and superficial gas velocity on the measured cluster properties were determined. The solid hold-up of the clusters was correlated to the horizontal length scales of the clusters to estimate the importance of each cluster size in terms of the mass fraction of solids residing in clusters of a specific width range. In the bottom region up to 0.2 m from the air distributor, about 5% of the solid mass in clusters was found to reside in structures with horizontal length scales less than 1 cm while at 1.2 m height the corresponding share was about 10%. This information is relevant for determination of the mesh resolution required to describe all the significant cluster sizes in CFD simulations. The findings of the study on the width distributions of the clusters and the solid concentrations inside the clusters may also be used as a point of comparison with corresponding measures observed in simulations, by which a quantitative validation of CFD results could be made. (C) 2015 Published by Elsevier B.V.