The earth would be a frozen lifeless rock stuck to the side of a much larger one. In darkness.
what if the sun was the size of the moon and the moon is the size of the sun?
It would make for a hell of a full moon!!!
Assuming that the planets are still part of the solar system, they would orbit around the moon instead of the sun. It would also have a stronger gravitational pull, as it is a lot denser than the sun is. The planets would orbit closer to the moon than they do the sun, and their orbits and rotations would be different. If you know anything about comets, you would know that as they get closer to the sun, they melt, creating tails. The comets would not have tails when they came close to the moon, and their orbit would be completely different to what they are. The sun's light would not reach nearly as far as it does, and the sun would likely not be the same type of star that it is, but even assuming that it is, and that it orbits around the Earth like the moon does, the Earth would theoretically be a lot hotter than it is, due to the sun's proximity, and chances are, that we would not be here to destroy the universe.
You wouldn't know the difference?
We would never need flashlights on most night.
But on the other hand... There wouldn't be life as we know it.