Short answer; pretty darn big. Astronomer Carl Sagan became very well known for his repeated use of the phrase ‘billions and billions’ when describing the number of galaxies in the universe. The same figure can be used the describe the number of stars in a single galaxy. However, even if we could count all of the galaxies and measure them, our figure would still be wrong, because the universe is expanding.

Astronomers using very powerful telescopes have come to realize that the entire sky is filled with galaxies too far away to be seen except by powerful telescopes. Each of those galaxies contains ‘billions and billions’ of stars, and most of those stars are much larger than our own star, the Sun. It would take an astronomically long time to calculate the number of visible galaxies in our sky, let alone their respective sizes.

To put things in perspective, consider our galaxy; the Milky Way. Astronomer Harlow Shapley estimated that the milky way galaxy is between 80,000 and 120,000 light years across. Our planet is located in one of the spiraling arms, near the outermost edge, and traveling at an approximate speed of 155 miles per second (over nine thousand miles per hour) it still takes the earth between 200 and 250 million years to make a single circuit around the galactic center. And that’s only one galaxy; there are billions!

The biggest problem facing a measurement of the universe, however, is not the size of galaxies. The problem is that all of the galaxies we can see are moving away from us, and we are moving away from them. When scientists look at time lapsed images of things which are moving away from them, the visible light spectrum shifts toward ‘red’. They have seen the same effect on astronomical images they have seen in the night sky. This tells them that the objects they see in their telescopes are moving away from the planet Earth. This effect is known as the ‘red shift’, and shows that the universe is expanding in all directions.

Because of this expansion, it is impossible to give an exact figure for the size of the universe. The best answer we can give is that it is infinitely large; it goes on forever in all directions.