The size of the universe may well be infinite, but if we consider that part of the universe that is visible to us (the observable universe), then:
The diameter of the observable universe is: 93 billion light years across.
Note that we say diameter because the shape of the observable universe is a sphere centered on the point of observation, the earth.
A light year is the distance light travels in 1 year. About 6 trillion miles.
(Light travels at 299,792,458 meters per second, or 186,000 miles per second.)