How big is a star?
That's a big question, because stars vary enormously in size!
The smallest are white dwarf stars, and are similar in size to Earth. When the core of a red giant star has exhausted all its fuel and shed all the gas it can, it's called a white dwarf and is considered "dead" because the atoms inside it can no longer fuse together to produce energy.
Next come brown dwarf stars, 15 to 75 times the size of Jupiter, so between the size of a planet and a small star and are not big enough to sustain the fusion of hydrogen. Instead they fuse deuterium or, in the case of the most massive, lithium. All brown dwarf stars observed so far are part of a binary system.
Third comes a yellow dwarf star. Like our Sun! This is a star in the stable part of its life-cycle (called a "main -sequence" star.)
Now we leave the dwarf stars and come to the blue giant stars. A blue giant is 25 times larger than our Sun and puts out as much as 10,000 times more energy.
After this is the orange giant. This is a star that has swelled and brightened a great deal as it approached the end of its life. Aldebaran is an orange giant and is about 30 times the size of our Sun.
Now we've reached red giant stars. This is a star reaching the end of its life. It grows to about 400 times its original size, cooling and glowing redder. Our Sun will become a red giant in about 5 billion years.
And lastly blue supergiant stars. These are 100 or more times bigger than our Sun. They "live" for only a few tens of millions of years, after which they run out of hydrogen and start to use up the helium in their cores, which cause them to burn hotter and brighter. The heat and pressure produced causes the star to swell and soon (in cosmic terms), the star will become a supernova.