The flow pattern in a circulating fluidized bed (CFB) is characterized by clusters of particles. The length-scale distribution of solid clusters was analyzed both horizontally and vertically from a number of experiments carried out with Geldart B particles in a two-dimensional circulating fluidized bed. Cluster dimensions were found to be dependent on solid loading, superficial gas velocity and location in the bed. Large length scales were found to be typical for the bottom region, above which more narrow dense clusters are common. The majority of clusters in the upper part of the riser have a width less than 20 mm, which means that in CFD simulations of circulating fluidized bed's the mesh resolution should be finer than 5 mm to accurately reproduce the clusters. However, a significant fraction, of the order of 10-15% of the measured cluster widths, was below 2 mm. This is about ten times the particle diameter which is the mesh spacing recommended in the literature for CFD simulation of CFBs. The findings of the study on the width and height distributions of the clusters may also be used as a point of comparison with corresponding measures observed in simulated systems, by which a quantitative validation of CFD results could be made. (C) 2014 Elsevier B.V. All rights reserved.