We report colloidal growth of PbSe nanosheets and finely size-tuned PbSe nanocrystals (NCs) via simple control of reaction parameters. The approach involves slow injection of precursors with excess amounts of oleic acid. Retarded growth, due to both the slow supply of precursors and the surfeit of oleic acid, causes attachment of PbSe NCs through the (110) planes, which are more reactive than the (100) facet, into a two-dimensional geometry. In contrast, such attachment processes can be prevented by impurities, e.g., Cd chalcogenide (CdSe or CdS) NCs dispersed in chloroform. For instance, the slow injection of Pb and Se precursors into a reaction solution containing Cd chalcogenide NCs results in the growth of spherical PbSe NCs, as the Cd chalcogenide NCs hinder the PbSe nuclei from merging via (110) planes. Compared to conventional rapid-injection methods, PbSe NCs grow slowly, which enables fine control of NC size. Ab initio calculations suggest that Cd precursors strongly bound on the surface of PbSe NCs may impede nanosheet formation and may slow PbSe NC growth.