Answer 1:
I do not think we know for sure the size of the
universe yet, because we can only probe a small
part of it. Whether the universe is finite or
infinite, depending on the "curvature" of the
universe.
Imagine we look at a globe, whose surface is a
two dimensional sphere. The global has a finite
surface area, because it has a positive curvature.
If we reduce the curvature of the globe, its size
gets bigger and bigger. And if the curvature is
zero, the globe becomes infinite, because
effectively the globe becomes a flat two
dimensional plane without boundaries.
The universe could be thought of as a three
dimensional sphere. We can estimate the size of
the universe (if it is indeed finite), by
measuring its curvature. This can be done even
though we cannot see the entire universe. So far
the understanding is that, the curvature of the
universe is nearly zero, which means that the
universe may as well be infinite, or at least much
bigger than what we can see.

Answer 3:
We can see out into the universe a
distance of about 14 billion light years. This is
because the universe is only 14 billion years old,
and so light has had only 14 billion years to
travel. The real universe is probably much
bigger than this, and may indeed be infinite.
Within the 14billionlightyear radius that we
can see, there are about 50 billion galaxies, and
the average galaxy contains about 100 billion
stars.
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