Could a human being be the size of an ant?

Unlikely. Humans are mammals, and no mammal has ever been found that is that small. There are several reasons for that. One, our internal organs may not be able to scale down to such a small size. Another reason is that the smaller an object, the more surface area per unit volume it has. Since heat loss is proportional to surface area, it means such a small animal will gain or lose heat very quickly. Since mammals generate body heat internally to keep our temperatures high, that means we can lose heat quickly to the environment if the environment is colder than our body temperature, and that means our hearts would have to beat very fast to keep up. Mammals the size of a mouse has a heart rate of 500 beats per minute, and they only live 2-3 years maximum as a result. A mammal the size of an ant may need to have a heart rate that beats thousands of times a minute, and the life span of such an animal would be a fraction of a year. That would not allow many animals enough time to grow and reproduce before dying.

Not to a human eye or an ant eye

In the movies, yes. Or standing next to a dinosaur, then in comparison, yes,

we really have no way of knowing how big the universe is...it could be the size of a tennis ball for all we know...

No......because of gravity

No. Ants don't exactly have brains. (They have something similar, but much simpler.)

no. there is an Inverse-square law (if I have the right term) that controls proportions as you increase or decrease size. The human body as is would not function at that size. The brain would have much less capacity, the muscles would be oversized, etc.

look at the legs on an elephant as an example.

Prepare yourself.. I'm about to get scientific......

..........

No.