Wo says, that Aliens have our size? The size of humans? When they are two heads taller the are already considered monsters, right?

We know insects, mice, blue whales. All of them orders of magnitude bigger or smaller than us. So what if aliens are the size of a thimble and their spaceships are the size of an average action drone? This only happened in one SciFi universe so far (AFAIK). John Scalzi's "Old Man's War"

The smaller a biological body is,

• the smaller the tools and vehicles are,
• the easier the ascent into space,
• the relatively more resources are available on the planet,
• the less energy is needed to survive
• the more living space is available

So there are evolutionary advantages in being small. So universe, give us tiny aliens.