The smallest molecules that allow water to remain fluid are hydrogen and oxygen atoms, which bond together to form water molecules.
These specific molecules are necessary for water to be fluid because they have a polar covalent bond, meaning the electrons are not shared equally between the atoms. This makes water a polar molecule and allows it to have unique properties such as surface tension and the ability to dissolve other substances.
The size and shape of water molecules do not directly affect its fluidity, but rather the interactions between the molecules. The polar nature of water molecules allows them to form hydrogen bonds with each other, which gives water its fluidity and ability to flow.
No, water cannot be fluid with smaller or larger molecules. The unique properties of water, such as surface tension and the ability to dissolve other substances, are a result of the specific size and shape of its molecules.
If the molecules in water were not able to form hydrogen bonds, water would not be able to remain fluid and would instead become a gas or solid at room temperature. This would greatly impact the Earth's ecosystems and make life as we know it impossible.