How big do scientists estimate the universe was before the big bang occurred?

We don't even know how big our own universe is, or what's beyond it. The big bang is considered the beginning of time itself, though there must have been something before it. Perhaps our brains are not even developed enough to understand the complexity of what was before, if there was even anything. There is no way to find out if there was even a universe before the big bang, so no one can answer your question.

I won't go into further detail because doing so would result in an excessive amount of text. However, if there was a universe that existed before ours, I believe it to have been several millions or even billions of times larger than this one since both would have required a big bang.

According to scientific estimates, the universe was incredibly small prior to the Big Bang and may have been a singularity with infinite temperature and density.