The largest group of virus is

The largest group of the virus is Closteroviridae.
Closteroviridae is a family of viruses. Plants serve as natural hosts. Viruses in the family Closteroviridae are non-enveloped, with flexuous and filamentous geometries.
The diameter is around 10 - 13 nm, with a length of 950 - 2200 nm. Genomes are linear and non-segmented, bipartite, and around 20kb in length.
The family Closteroviridae contains more than 30 plant viruses with flexuous, filamentous virions, with either mono- or bipartite (one tripartite) single-stranded positive-sense RNA genomes.