A reasonable way to calculate the size of the universe is to first calculate its age, and then assume expansion at the speed of light. Age can be determined by

A reasonable way to calculate the size of the universe is to first calculate its age, and then assume expansion at the speed of light. Age can be determined by a number of factors, including calculating the age of the oldest stars we observe, using objects such as Cepheid variable stars (whose brightness is a function of time), and measuring relative velocities of distant galaxies to then figure out when they started moving (Big Bang).