In this article, the digital image analysis (DIA) technique has been used to study the bubble size and bubble size distribution evolution in a pseudo-two-dimensional gas-solid fluidized bed. In addition, a numerical discrete bubble model (DBM) based on the bubble-bubble interactions was developed and the model predictions were compared with the experimental data. The developed model can predict the bubble size and bubble size distribution through the bed height and the lateral distribution of bubbles. Comparison between model predictions and the result of DIA measurements shows that bubble breakage has a significant effect on the bubble size distribution especially at heights far from the distributor plate. Moreover, lateral bubble distribution of the bed was investigated.