Planets that are smaller than Earth are much more likely to have their hydrogen stripped away by their host star, leaving them without water.

Planets that are larger than Earth are likely to have their growth accelerate into gas giants. Mass distribution of planets shows a gap in sizes between about 1.5 and 2 times the size of the Earth.

Life requires complex chemistry. Complex chemistry can't happen in the dense, hot cores of gas giants, because the heat will destroy any interesting chemicals. Complex chemistry can't happen in the solid phase, because nothing moves around very much. Even though complex chemistry can happen in the gaseous phase, life can't start, because the simple life precursors can't bind themselves. So it requires a liquid. Liquid either means water, so not small planets, or temperatures found only outside the habitable zone, which means solvents other than water, which are unlikely to host life for a variety of reasons.