When the Sun enters its red giant phase, it will expand to become roughly 100 times its current size.

Here's a breakdown:

* Current Radius: The Sun's radius is approximately 695,000 kilometers (432,000 miles).

* Red Giant Radius: As a red giant, the Sun's radius will increase to about 1 AU (Astronomical Unit), which is the average distance between the Earth and the Sun. This is equivalent to about 150 million kilometers (93 million miles).

Important Notes:

* The Sun will not engulf the Earth directly during this phase. However, the Sun's expanding outer layers will consume Mercury, Venus, and likely Earth as well.

* The expansion process will take millions of years, giving humanity ample time to relocate or find a way to survive.

It's worth mentioning that the Sun's exact size during its red giant phase is subject to some uncertainty, with estimates ranging from 80 to 160 times its current size.