Firm size follows Zipf's Law, a very fat-tailed distribution that implies a few large firms account for a disproportionate share of overall economic activity. This distribution of firm size is crucial for evaluating the welfare impact of macroeconomic policies such as barriers to entry or trade liberalization. Using a multi-country model of production and trade in which the parameters are calibrated to match the observed distribution of firm size, we show that the welfare impact of high entry

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... sts is small. In the sample of the largest 50 economies in the world, a reduction in entry costs all the way to the U.S. level leads to an average increase in welfare of only 3.25%. In addition, when the firm size distribution follows Zipf's Law, the welfare impact of the extensive margin of trade -newly imported goods -vanishes. The extensive margin of imports accounts for only about 3.5% of the total gains from a 10% reduction in trade barriers in our model. This is because under Zipf's Law, the large, inframarginal firms have a far greater welfare impact than the much smaller firms that comprise the extensive margin in these policy experiments. The distribution of firm size matters for these results: in a counterfactual model economy that does not exhibit Zipf's Law the gains from a reduction in entry barriers are an order of magnitude larger, while the gains from trade liberalization are an order of magnitude smaller. JEL Classifications: F12, F15