The moon is about 400 times smaller than the Sun. But is also 400 times closer than the Sun. Which makes them about the same size if looking from Earth which creates a total eclipse. If the Sun was any bigger it could not create a total eclipse.
Yes
That being said, since the Sun is the only light source and the moon will cover the entire Sun looking from Earth, there should only be 1 shadow. Not different shadows. And it should be bigger than the size of the moon. So that means it’s he Moon has to be smaller than 70 miles.
No. Because the sun is not a point light source, it emits light from all points and in all directions - obviously,that's why we can see the whole disc of it.
This is quite hard to explain and impossible to show to proper scale but I've had a go at demonstrating the concept.
In this diagram the "sun" is 10 times bigger than the "moon" and 10 times further away from the "earth". So they would appear the same size.
But you can see from the way the light rays travel that the eclipse that would form is smaller than the moon. I've shaded the area of totality.
And you can see the area of partial eclipse

Edit: I thought of another way of explaining it just as I woke up! It is
because the sun is 400 times bigger and 400 times further away that the path of totality (the umbra) is so small compared with the size of the earth. And it’s why total eclipses are rare. Think about it. If you line up a large distant ball with a smaller big one in such a way that the small one exactly fits into the outline of the big one then you won’t be able to see the big one, yes? That’s like being in the path of totality. Now what happens if you move your head? Now you can see see the edge of the big ball. That’s like being in a partial eclipse.
The point is, you don’t need to move your head much. And it would be the same if you moved the smaller ball slightly. What happens if you move the smaller ball a bit further away? Now you can see the outline of the bigger ball around the smaller one - and that happens with the moon too because the moon is not a constant distance from the earth, that’s called an annular eclipse.
The point is if a distant large ball is “x” times bigger than a small one and “x” times further away then the small one can “eclipse” the big one because they are the same angular size. But
only from one vantage point if anything moves then they don’t line up properly. That’s why the path of totality is small and moves across the earth as the earth spins and the moon moves.