Virus particles(virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 um.A common unit of measure for viruses is the nanometer. which is 1000 times smaller than 1 um and 1 million times smaller than I mm. Smallpox virus, one of the largest viruses, is about 200 nm in diameter(a bit smaller than the size of the smallest bacteria); poliovirus, one of the smallest, is only 28 nm in diameter (about the size of a ribosome)Virus particles (virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 μm. A common unit of measure for viruses is the nanometer, which is 1000 times smaller than 1 um and 1 million times smaller than 1 mm. Smallpox virus, one of the largest viruses, is about 200 nm in diameter (a bit smaller than the size of the smallest bacteria); poliovirus, one of the smallest, is only 28 nm in diameter (about the size of a ribosome)