I've heard that habitable world has to have at least 25% of Earth mass - but why?
Is there any way to sustain heat and internal magnet in a smaller world? (for example, internal nuclear reactor - could it work?)
What if I don't care about volatiles escape, but just want an easy-to-terraform world (is it possible to have world which requires only a few nuke explosions in right places and some plants to get breathable atmosphere within 100 years?)?