300 - 500 nm
||Young KD. The selective value of bacterial shape. Microbiol Mol Biol Rev. 2006 Sep70(3):660-703 DOI: 10.1128/MMBR.00001-06 p.665 left column 2nd paragraphPubMed ID16959965
|| Connon SA, Giovannoni SJ. High-throughput methods for culturing microorganisms in very-low-nutrient media yield diverse new marine isolates. Appl Environ Microbiol. 2002 Aug 68(8):3878-85  National Research Council Space Studies Board. 1999. Size limits of very small microorganisms: proceedings of a workshop. National Academic Press, Washington, D.C.  Rappé MS, Connon SA, Vergin KL, Giovannoni SJ. Cultivation of the ubiquitous SAR11 marine bacterioplankton clade. Nature. 2002 Aug 8 418(6898):630-3 DOI: 10.1038/nature00917PubMed ID12147485, 12167859
||P.665 left column 2nd paragraph: "Koch observes that the lower boundary of prokaryotic cell size is that which is “large enough to house the total amount of needed stuff” (ref 170). That is, the cell must have sufficient room to include all the nucleic acids, proteins, molecular complexes, and other gear required for survival and proliferation. By calculating the amount of space required to house this “needed stuff,” the lowest theoretical size for a free-living prokaryotic cell is estimated to be a sphere of 250 to 300 nm in diameter (primary source 227). This is very close to the size of the smallest bacteria observed in oligotrophic oceanic environments, these cells being tiny rods or coccoidal cells from 300 to 500 nm in diameter (primary sources 44, 227, 266)."