What is the size of red giant

Average sized stars like our Sun will reach the Red Giant stage at the end of their lives before becoming White Dwarfs.

Larger stars perhaps 8-10 times the size of our Sun will supernova when they die.

As a star burns, it converts hydrogen into helium. After most of the hydrogen is burned, the star starts to burn the helium. This creates hotter temperatures and causes the star to expand. The red giant stage is a process occuring over millions of years.

Larger stars not only sythesize hydrogen into helium but heavier elements like silicon, sulfur and iron. This causes the core to eventually implode under it's own weight. The pressure is stabilized by neutrons in the core producing an explosion. The neutron star left behind shines brilliantly and lights up the gas cloud it ejected thus giving it it's name supernova. There are two types of supernovae. The other type can be caused by a large white dwarf which accumulates enough material that it ignites a carbon fusion and explodes leaving nothing behind. In either case, supernova stages occur very quickly lasting only months. This is why it is rare to see supernova occuring, only the remnants of one.