What is the smallest a world can be & still have the geologic life of Io & Europa, before it can no longer pull itself back together again?

Astronomers were very surprised that the tiny moon Enceladus has guysers and geologic active. So nobody really knows how small a geologically active body can be. 

Europa has double the water of  earth but its frozen. When sun becomes red giant and earth is toasted to a crisp --- Europa can have habitable temperatures and liquid running water. 

However its unclear if oxygen to breathe will ever be there on europa. If plants can somehow be grown oxygen maybe produced. It's then possible to live there

But again all this is billions of years away

Io and Europa are geologically alive because they are tidally compressed by Jupiter's gravitational field.  Otherwise they'd be dead.  So it depends on the tidal forces on the moon or planet.

Won't know that until we find some sort of indication of life beyond Earth.