Si oze Viruses vary considerably in size. Although most are quite a bit smaller than bacteria, some of the larger viruses(such as the smallpox virus) are about the same size as some very small bacteria(such as the mycoplasmas, rickettsia, and chlamydia) Viruses range from 20 to 300 nm in diameterViruses vary considerably in size. Although most are quite a bit smaller than bacteria, some of the larger viruses (such as the smallpox virus) are about the same size as some very small bacteria (such as the mycoplasmas, rickettsias, and chlamydias). Size Viruses range from 20 to 300 nm in diameter