Although we don't know exactly how many stars are in the Universe, we can estimate based on how many are in our own galaxy, The Milky Way, and then multiply that by the number of galaxies in the Universe. However, we don't know how many galaxies there are either. There may be several hundred billion stars in our galaxy alone, and then there are also billions and billions of galaxies in the Universe. So, we can estimate there are trillions of stars out there- a number so big it is hard to imagine!
When you ask about how big the Sun is, if you are talking about distance from one side to the other (diameter), it is about 1,400,000 km, which is about 109 times larger than the Earth's diameter. If you are talking about mass, the Sun's mass is 1.99*10^30 kg, and the Earth's mass is 5.98*10^24 kg which means the Sun is about 333,000 times more massive than the Earth. About 1,300,000 Earths could fit inside the Sun!
Holly