peer reviewed ; The firm size distribution (FSD) is one of the most well-known economic empirical regularity in market economies. Its functional form, postulated in macroeconomic models with heterogenous firms, can be approximated by a parametric distribution. However, the parametric approximations proposed in the literature have long been contested due to the lack of unbiased databases and robust statistical methods. This paper adresses these shortcomings. First, a robust estimation method is proposed to test the fit of parametric distributions and to determine the one offering the best fit at different truncation thresholds. Then, by applying it to a comprehensive database of Belgian firms for the period 2006-2012, the results show that the lognormal distribution is a better approximation of the empirical FSD than the Pareto distribution. These results hold true at the aggregate, sectoral and regional levels establishing that the shape of the aggregate distribution is not an aggregation artefact arising from the potential distributional heterogeneity of sectoral or regional subsets. This empirical evidence is essential information for tracing the root causes of a plausible underlying stochastic model explaining firm dynamics.