I assume you are talking about straight needles? Sands gave you some good answers already. 12 inches is fine for many flat projects, but for things that are large like afghans and seamless sweaters you'll want circulars probably.
Most patterns give you needle size, gauge and suggested yarn or yarn weight. If for some reason there is no needle size it may depend on the yarn you choose. They don't usually tell you what type of needle since that is personal preference. For instance I use only circulars regardless of what the project is or what it tells me. I don't even own any straight needles anymore.