How Big Was The Universe When The Big Bang Started?
In the original version of the big bang theory, it was common for people to try to mathmatically and conceptially "rewind" the big bang expansion all the way back to a "point" or "singularity". But we now know that makes no sense. The universe cannot be rewinded to a "point". Because when the big bang expansion started, the observable universe was not a point, it was a sphere, same as now.
https://bigthink.com/starts-with-a-bang/small-universe-big-bang/
...But what if we went all the way back: back to the very beginning, and the very first moment of the hot Big Bang itself? Surprisingly, it doesn’t give us a singularity, where the Universe reaches infinite densities and temperatures at an infinitesimal size. Instead, there’s a limit: a smallest possible size that the Universe could have had. Here’s why that limit exists, and how we can figure out the minimum size of the early Universe...
The now observable universe was at least 1.5 meters in diameter at the beginning of the Big Bang expansion, and perhaps as big as a city. Today, we can say that the Universe got no hotter, at the hottest part of the hot Big Bang, than about ~10¹⁵ GeV in terms of energy. That places a cutoff on how far you can extrapolate the hot Big Bang backwards: to a time of ~10-35 seconds and a distance scale of ~1.5 meters. The Universe, at the earliest stages we can ascribe a “size” to it, could have been no smaller than roughly the size of a human being...It could still have been much larger, like the size of a city block or even a small city, for example