Developing countries typically exhibit small firm size, high dispersion of marginal productivity of factors across firms, and low trade-to-output ratios. They also tend to export particularly less to more distant and smaller markets. To rationalize these facts,
this paper develops a flexible, multi-country general equilibrium model of production and trade in which heterogeneous producers face both domestic size-dependent distortions (SDD) and costly entry into exporting. Since larger firms have greater export-market
participation, misallocation induced by SDD reduces the economy's overall market access, trade volumes, and gains from trade, reinforcing the contraction in aggregate total factor productivity (TFP). I explore the quantitative properties of the model calibrated
to firm-level and aggregate data from the manufacturing sector of 77 major economies. I find that productivity gains from reducing SDD are significantly larger when economies are open to trade. Enhanced firm selection and factor allocation across firms fully
explain this amplification, whereas the contribution from changes in firm creation is actually dampened by trade. Furthermore, cross-country variation in SDD explains
a substantial share of international productivity differences, but only when countries are integrated through trade.