Virus particles (virions)vary widely in size and shape.Viruses are smaller than cells,ranging in size from 0.02 to 0.3 um.Smallpox virus,one of the largest viruses,is about 200 nm in diameter; poliovirus,one of the smallest,is only 28 nm in diameter.Virus particles (virions) vary widely in size and shape. Viruses are smaller than cells, ranging in size from 0.02 to 0.3 um. Smallpox virus, one of the largest viruses, is about 200 nm in diameter; poliovirus, one of the smallest, is only 28 nm in diameter