I'm also curious as to what you have in mind here. I think the smallest plasmids are around 1300 base pairs... but 100?
My understanding -- see here for a preliminary cite -- is that we've seen replication and evolution in RNA molecules with fewer than 50 bases. (Basically, it's an extension of Sol Spiegelman's "monster" -- the full reference, which I have not read, is "30 years later - a new approach to Sol Spiegelman's and Leslie Orgel's in vitro evolutionary studies: dedicated to Leslie Orgel on the occasion of his 70th birthday, M. Eigen and F. Oehlenschlager, Orig. Life Evol. Biosph. 5-6 (1997), 437-457.")