Usually, for obvious theatrical reasons, “people from outer space” are just that: people like us. There are some deviations from this theme: Contact is one (the form of the alien is merely a representation it provides to the viewer), close encounters the same.
But what is the likely evolved form of a space travelling life? I think it will be very small, based on two assumptions. The first is that the new form doesn’t
have a way to violate e=mc2. The second assumption is that this travelling life actually makes its home in space, rather than merely moving from place to place. In this latter case, either most “individuals” might be transported as DNA and then grown using mass from the destination planet, or perhaps constructed from information, if that is possible.
Under these two assumptions, I think that a space-travelling species will be very (very) small: primarily because mass is expensive to accelerate and decelerate, so it seems that any species that doesn’t will be very tiny.
It might be argued that the living cargo is a small fraction of the mass of the ship, and this may be true. But people form a large fraction of the payload, and the total mass of the ship is driven by payload.
Preferably, a space traveller would weigh mere grams: Most of the body should be done away with. And the brain shrunk. this raises questions of how small a computationally sophisticated brain can become. Partly getting small helps you get small: lots of weight is myelinated fibres going long distances, and blood vessels going long distances.
Partly some simple design choices can reduce mass substantially: principally reducing the large volume of water in the brain. Gas could be used as an insulator instead of fat. Gas can also work as a cushion, instead of csf, and large volumes such as the lateral ventricles can be reduced massively.
But, the fundamental building block is the neuron. As neurons are single cells, it is unclear if they can be reduced in size without decreasing reliability.
If the brain can only be shrunk by, say, half, lots of pressure must be placed on the rest of the body to reduce in size.
Hmm. Not unlike the Close-encounter creatures.