I'm new to this forum, but I've been reading here a lot over the past several days. I'm planning on starting a sci-fi universe for a gaming group I'm in and for a book that I want to write. I've been doing a lot of research and I want the first planet I'm designing to exist in the Castor system, what I've read(though it may be outdated) says that CastorA has a habitable zone ~6AUs out and CastorB has a habitable zone ~4AUs. My question is how large would/could these planets be? I've done a lot of reading and I understand that it is a function of mass, composition, and spin and the center has to be iron, however I can't really find anything like equations. I want the planet to have a roughly Earth-like composition, though possibly slightly less metallic, so as to explain the reliance on non-technological things rather than being hyper-advanced. I would also like to know if it is possible for a planet of composition like that to support(earthlike) life and have a surface area twice or maybe three times that of Earth. Any help would be greatly appreciated, thank you.