Today, do scientists believe the Universe was the size of an atom, or even larger?

The size of the Universe is unknown. We can only see a small part of it, 13.7 billion lightyears in radius. It is certainly larger than that, likely 10^20 times larger or even more.

So when it is said that the Universe was the size of an atom, what is meant is that the part of the Universe that is now visible to us was once the size of an atom. We know the equations of motion governing the Universe (the Friedmann solution to the Einstein field equations) so we can, in fact, extrapolate back that far. (Just as you know how a balloon behaves, and you know that because of the nature of rubber, etc, that an expanding balloon was smaller, but there is a point beyond which extrapolation is not sensible.) Extrapolating backwards about the Universe does fail, back at the first tiny fraction of a second. It is remarkable that physics works backwards as far as that. We don't know what happened before the point at which physics fails us. We really don't know about "time zero", only time 10^-33 seconds. We don't know how big the whole Universe was at any time, we only know about the size of the part within our event horizon. It is possible that the Universe is infinite in size and was always infinite. Anything about prior Universes, or the insides of previous black holes is just speculation.

So far the evidence we have found indicates that the Universe started as a dimensionless point. I'm not smart enough nor do I understand the reasons why other scenarios have been ruled out. One of them is the distribution of light elements in the Universe, which was predicted by the Big Bang theory, and is impossible to explain with alternate theories.

Secondly, your idea that the Universe originated with a black hole in another Universe is, well, comic book science. You won't find that idea anywhere in mainstream cosmology as far as I know. Try brane cosmology:

http://en.wikipedia.org/wiki/Brane_cosmology

the great Hadron Collider is a 27-kilometre lengthy round particle accelerator on the CERN experimental facility close to Geneva which will smash protons into one yet another at unbelievable speeds attempting to copy in miniature the activities of the large Bang. If the theories are perfect suitable, the gadget will create tiny Black Holes that evaporate and probable even locate debris that provide data that the three dimensions familiar to mankind are only a fragment of those who surely exist. With all new great jumps in technologies the doomsayers seem and at present they spout concerns that by utilizing adventuring into the unknown and coming up tiny Black Holes, the gadget ought to even threaten to damage the planet. The probability of that's on the point of 10 to the minus 40!!! Professor Stephen Hawking has suggested that without this attempt guy will be doomed to die by potential of no longer studying adequate to enable mankind to learn the technological understanding to manage to stay in area and bypass to different planets at the same time as this Earth is ultimately threatened!!

No one knows. For all we know space is infinite. We know now though that its expanding. So rewind that expansion. A millisecond after the big bang the universe is still infinitely large. A femtosecond after the big bang the universe is still infinitely large. However, at the big bang, the universe's size is 0. That's the singularity everyone talks about.

It is an question that can't be answered since there is no test that can prove it right or wrong. Therefore if there is no test, there is no scientific method for it, and it is not science.

you know, it's a theory. not been proven yet. so you argue with people about his. there is no right or wrong answer, until it's been proven.