The main reason I'm an atheist

[Wryetui]

How does the size of the universe tell us God does not exist?

The size of the universe doesn't say one way or the other whether or not any gods exist, it does however beg the question: if humans are the focus of your creation, why design a universe in which humans are so situated as to appear to be utterly insignificant?

Now someone who actually answered! Thank you.

However, the size of the universe has nothing to do with the existence of God. And that is what the OP stated, that, as the universe is as big, it cannot the possible (somehow) that God exists. I have seen in this something incorrect and unrelated.

We could talk about how small humans are in relation to the universe and other things like that, but that would be just speculation since nowhere in the Sacred Scripture or the Sacred Tradition it is stated why the Creation is so vast (perhaps because such an information would be quite useless to the people living before Christ, for example), but however big or small is the universe, it has nothing to do with God's existence.