While self studying cosmology as well as reading around I can't find a satisfactory answer to this question. My thinking is that in principle universe could have started already infinite or as a thiny speck.
The first case is as to say that I can imagine a collection of thiny speck or it is wrong? This picture indeed let me think of space as already existing unless those infini speck of something can imagined as not having volume .
But at least they should have had one at about the Planck scale.
Conversely, if I think of only one speck, I do understand that it could have generated the observable universe as well the "outer" one (via inflation, right? ) but I have troubles to see how the cosmological principle can be used in very very far parts of the universe. In other words why it should/could have a size exceeding those compatible with inflation (this latter is a mind blowing factor, but not infinite).
I am not arguing about currently standard ideas, I try to visualize them .
Edit: or simply it was infinite but space did have to start its expansion? This and its consequences are impossible to visualize but conceptually could make sense. However, there should be an horizon problem at enormous scale exceeding the inflation range.... really confusing