The smallest RNA viruses in terms of genome size are the smallest of the retroviruses (eg Rous Sarcoma Virus); genome 3.5 kb, particle diameter 80 nm. The smallest DNA viruses are the hepadnaviruses (eg hepatitis B) (3.2 kb, 42 nm). The parvoviruses have smaller capsids (18-26 nm), but a larger genome (5 kb). Viroids are infectious agents of plants that are comprised of a single small molecule of RNA. They lack a capsid and are not considered to be viruses. The smallest DNA bacteriophage is phi X174 which I believe is larger than Hepa B. RNA phages typically contain about 4 kb. I don't believe there are any plant viruses smaller than what I've noted.