How does the physical diameter of a star change during the red giant phase?
Answer
It can increase by 100 to 1,000 times the original size
During the transition from the main sequence to the red giant phase, the star undergoes a dramatic physical expansion. Depending on the star's initial mass, the diameter can swell to be 100 to 1,000 times larger than it was previously. This massive expansion is sufficient to engulf inner planets if such a process were to occur in our solar system.

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What Are Red Giant Stars? - YouTube
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