Could aliens be microscopic?

Yes. If extraterrestrial life exists, it could absolutely be microscopic.

Many scientists think microbial life is more plausible than complex alien animals or plants, because microbes need far fewer resources and life on Earth was microscopic for a very long time before complex organisms appeared. On Earth, many microbes can survive extreme conditions such as heat, acidity, cold, and even the vacuum of space.

Scientists are also looking for possible signs of microbial life on Mars and in places like Europa and Enceladus. Current Mars missions are searching for biosignatures of ancient microbial life.

That said, no definitive extraterrestrial life has been found yet, microscopic or otherwise.