Aliens are probably enormous and weigh over 300 kgs!

Aliens, if they exist, would be much larger than humans, weighing over a massive 300 kilogrammes, a cosmologist in Spain has calculated.
Fergus Simpson, a scientist at the University of Barcelona, has estimated that the average weight of intelligent extraterrestrials would exceed 300 kilogrammes.
The argument relies on a mathematical model that assumes organisms on other planets obey the same laws of conservation of energy that we see here on Earth - namely, that larger animals need more resources and expend more energy, and thus are less abundant.
There are many small ants, for example, but far fewer whales or elephants.

Thus, throughout the universe, as is the case on Earth, there are likely more small animals than large ones, said Simpson.
Since the number of planets inhabited by relatively small animals would outnumber the amount of worlds where large ones predominate, it is most likely that we find ourselves on a planet with relatively small animals - and are ourselves probably one of the smaller intelligent beings, Simpson added.

Simpson said that a certain minimum size is probably necessary for intelligent life, and he used the range of body sizes found in animals on Earth to come up with the most probable distribution of organisms on other planets, 'Newsweek' reported.

Seth Shostak, a researcher at the SETI Institute, said the study reminded him of previous work he had done suggesting that any extraterrestrials we might eventually find would be on the larger side since bigger animals live longer, and an organism with a longer lifespan might be more likely to develop the kind of technology necessary for making contact with humans.
Shostak said the research is "interesting" but cautioned that there is no concrete data to work with.
He also said that while humans are not the Earth's biggest organisms it is their thumbs and upright stance - not their body mass - that has allowed their intelligence to blossom.

"Polar bears are large but do not write great literature and build radio towers and a lot of that is probably because they are walking around on all fours," Shostak said.