If the universe has a finite size, and is the only thing in existence, then its size can only be described as relative to itself and the things in it. In which case its actual size doesn’t matter. Just like when people ask “what is the universe expanding into?”, its a non sequitur because it’s expanding relative to itself and nothing else. But this is like asking how many angels can dance on the head of a pin. Theoretically the known universe was once far smaller than a single atom, and expanded to the size of a grape fruit in the tiniest fraction of a second. All the matter and energy in the universe phase changed at this point from the quantum vacuum. Is anyone really willing to say it’s impossible for God to have done this?
The materialists would.