What determines how large a star gets?

Answer 1

Their mass. If its mass is nearly like of sun it will become red giant. If its much bigger, it will become super giant

Answer 2

The mass of a star primarily determines its size. Stars with greater mass will be larger, as there is a balance between internal pressure from nuclear fusion and gravity pulling the star inward. Higher mass stars have more fusion reactions occurring in their cores, creating more pressure to counteract gravity and resulting in larger sizes.