There are living prokaryotes (Bacteria and Archaea) that have cell sizes that range from 0.02-400 microns3. Over this tremendous range, various abilities to cope with the environment are needed. This review attempts to formulate some of the problems and some of the solutions. The smallest size for a free-living organism is suggested to be largely set by the catalytic efficiency of enzymes and protein synthetic machinery. Because of fluctuations in the environment, cells must maintain machinery to cope with various catastrophes; these mechanisms increase the minimum size of the cell. On the other hand, the largest cell is reasonably assumed to be limited by the ability of diffusion to bring nutrients to the appropriate part of the cell and to dispose of waste products. To explore the limitation imposed by diffusion, analysis is developed of diffusion processes through stirred and unstirred media, diffusion through media that contains obstacles, and the effect of size and shape.