User blog comment:WC-83/Where ever are we at?/@comment-23784953-20130831165358/@comment-10390252-20130831171633

A planet's size determines a lot about its physical properties. It must be broadly the same size as Earth because it has a similar land/sea ratio. Denser or larger planets would have deeper oceans and denser atmospheres that would make it much less likely as a human habitat. If it was lighter, there would be far less free water and a thinner atmosphere, making it unlikely that large avians could exist as it would be difficult to generate aerodynamic lift.