How big will our sun gets when it becomes a red giant?
It is not certain exactly how big the Sun will get when it goes into its red giant stage. It will envelope both Mercury and Venus.
It may stop short of the Earth's orbit. Even so, the Earth will be so close to the Sun that it is hot enough for rocks to melt.
It is also possible that the Sun could expand out beyond the Earth's orbit or even to Mars' orbit with is 1.5 times further from the Sun than Earth. If this happens and Earth is inside the Sun, friction with gasses will slow the Earth and it will fall into the Sun.