Your question is a bit open ended, so I will attempt to address a couple things you could be asking :)

Are you talking about what would the simplest RNA have looked like from an evolutionary perspective? That I can't answer, unless we were able to re-create life in that way artificially there isn't any way for us to know.

If you are asking how small can an RNA strand be and still be able to perform a chemistry function, the smallest ribozyme I know of has 13 RNA nucleotides (Source), and functions when in the presence of a Mg²⁺ ion to cut other RNA's into pieces.