Where intelligent life forms come from all possible stops on the evolution scale, they are fairly consistent in size. The reason for this rule is simple - on the inevitable encounters, intimidation would be minimized. Thus whether a life form is similar to an insect, fish, octopus, snake, lizard, monkey, bird, or hominoid they have all been genetically engineered to be of the same approximate size.
Within these parameters there is great variation on other facets, however, as the life form must live on the world they evolved on, and this cannot be ignored. Thus, on those worlds where the mass of the planet is such that gravity exerts a great pull, the life form will evolve with enough strength in bone and muscle to move about. Thus the giant hominoids from the 12th Planet are large, heavy boned, and strong in comparison to humans, as their planet exerts almost twice as much gravity pull as the Earth. Thus we, the Zetas, are stronger than humans as our planets also had more mass and resulting gravity pull. Where skinny, our bones are bolstered by wrap around ribs and joints that move in fewer directions than human joints, thus preventing strain.
The evolution of a life form and the home planet determines to some degree where the life form can travel and work. Beyond the obvious differences in ability to tolerate high radiation from a hot sun or the need to breathe air or water or the base chemistry of the DNA, there are issues of gravity tolerance. We, the Zetas who have chosen work on the Earth's Transformation, cannot return to our home planets because the gravity would crush us. We have adapted to our new environments. Likewise it is known to your scientists that long space voyages without the semblance of Earth's gravitational field would doom your astronauts to a lifetime in space.
Beyond approximate size, the life form may be carnivorous or vegetarian or if the planet is lush with watery nutrients even simply absorb nutrients through the skin as many single cell organisms do. They may have teeth or no teeth, eyes or no eyes or multiple eyes, skin that is dry and tough or covered with slim or oil, bones as an interior or exterior skeleton or no bones at all, able to generate their own sugars as do plants or be completely dependent upon food located and eaten, breathing through lungs or gills or absorbing air or water components through the skin, covered with feathers or fur or scales or without an outer garment. Life forms come in all shapes, and humans embracing the Awakening should be mentally and emotionally prepared to encounter them. Bear in mind that they think of the human form as strange, too.