Depending on the ultimate shape of the cosmos, the entire Universe was smaller than today's visible bubble exactly 13.79 billion years ago—or literally never.
Understanding why requires separating two very different concepts: the observable universe and the entire Universe. The observable universe is the spherical bubble of space currently visible from Earth. Because the universe is 13.8 billion years old and space is expanding, the light from the farthest edges of observation has been stretched along the way. Today, that bubble of observable space is roughly 93 billion light-years in diameter.
But the observable universe is just the part of the cosmos whose light has had enough time to reach telescopes on Earth. The entire Universe extends far beyond that boundary.
Current cosmological data heavily favors the idea that the universe is spatially infinite, stretching out forever in every direction. If the universe is infinite today, it has always been infinite. It did not start as a finite speck that grew into infinity. When cosmologists describe the early universe as infinitely dense, they mean that an infinite expanse of space was crammed with matter and energy at extreme densities. As the universe expanded, the distance between any two points increased, but the total volume was always infinite. Under this standard model, the entire Universe was never smaller than the current 93-billion-light-year visible bubble.
However, some models suggest space might be closed—meaning a traveler going far enough in a straight line would eventually loop back to the starting point, much like walking around the surface of the Earth.
Based on measurements of the cosmic microwave background by the Planck space telescope, space is extremely flat. If the universe does loop back on itself, the curvature is so slight that the entire Universe must be at least 250 times wider than the observable universe.
Assuming the universe is exactly that minimum possible size, physicists can calculate when that colossal volume was squeezed into a space just 93 billion light-years across. Rewinding the expansion until the universe was 250 times smaller corresponds to a cosmic redshift of about 249. At this redshift, the universe was heavily dominated by matter and had been expanding for only 3.5 million years.
If the universe is finite, the entire cosmos was smaller than today's visible universe 13.79 billion years ago, back when it was just 3.5 million years old.