This paper presents an analytically tractable model of balanced growth that aligns with the observed size distribution of firms. The model incorporates firm-specific productivity improvements, selective survival of successful firms, and imitation by new entrants. Growth is driven by productivity shocks, selection of successful firms, and imitation. The model generates balanced growth and matches key features of the firm size distribution, including the Pareto-like tail with a slope of -1.06. The model explains the observed firm size distribution through entry and fixed costs, and the difficulty of imitation. Calibration based on U.S. data suggests that about half of output growth is due to selection. However, the implied variance of productivity shocks is unexpectedly high. The model also considers endogenous growth through imperfect imitation, leading to a tail index slightly above 1. The model shows that the size distribution is influenced by entry costs, fixed costs, and the productivity of new entrants relative to incumbents. The paper concludes that the observed firm size distribution can be explained by a combination of selection, imitation, and productivity shocks, with the tail index converging to 1 as entry costs increase relative to fixed costs. The model also highlights the importance of imitation in shaping the firm size distribution and the role of entry costs in determining the growth rate of the economy.This paper presents an analytically tractable model of balanced growth that aligns with the observed size distribution of firms. The model incorporates firm-specific productivity improvements, selective survival of successful firms, and imitation by new entrants. Growth is driven by productivity shocks, selection of successful firms, and imitation. The model generates balanced growth and matches key features of the firm size distribution, including the Pareto-like tail with a slope of -1.06. The model explains the observed firm size distribution through entry and fixed costs, and the difficulty of imitation. Calibration based on U.S. data suggests that about half of output growth is due to selection. However, the implied variance of productivity shocks is unexpectedly high. The model also considers endogenous growth through imperfect imitation, leading to a tail index slightly above 1. The model shows that the size distribution is influenced by entry costs, fixed costs, and the productivity of new entrants relative to incumbents. The paper concludes that the observed firm size distribution can be explained by a combination of selection, imitation, and productivity shocks, with the tail index converging to 1 as entry costs increase relative to fixed costs. The model also highlights the importance of imitation in shaping the firm size distribution and the role of entry costs in determining the growth rate of the economy.