How do astronomers explain the size of the universe?
They use the red shift to approximate it.
When it was discovered that the universe is expanding and the expansion is accelerating, scientists had a method via light/Doppler theory to measure distances in the galaxy. The theory is quite simple. When a light emitting object speeds towards you the light it emits compresses towards the blue end of the light spectrum. When that same object is moving away, the light waves lengthen towards the red end of the spectrum, hence the "red shift." Next the take the light of a particular type of star, Cepheid, as a baseline of candle power, a measure of the strength of light observed. The Cepheid is very stable so using these stars as measuring points, measuring their candle power plus the amount of red shift they can approximate the distance of stars and galaxies.