Hi, nice to see new thinking here. Have you read all of the posts in this thread? (only to stop myself repeating).The question is possibly (likely?) not a good question. You would have to reside outside of the universe to "start" your clock at the big bang. At least, that is what seems to be true, according to models consistent with the believed conditions of our universe.
To explain:
Our universe likely has a "boundless" condition. This applies to spacetime. Meaning, should you travel backwards in time, you would never, not ever, reach a "beginning".
Hawking struggled with this idea, until he finally (shortly before his death) formulated a mathematical explanation for this involving imaginary time.
You - "The question is possibly (likely?) not a good question."
I'm not wholly convinced it's unanswerable yet. So far we can go back to 10^-43 seconds - pretty good, so if our understanding of physics improves with quantum gravity etc, and more evidence is gathered, then who knows, it may be possible to determine the initial size. There are already good estimates for the size of the observable universe. If you can then demonstrate the whole universe is expanding in proportion to the observable universe, then it's possible.
You - "You would have to reside outside of the universe to "start" your clock at the big bang."
No problem for me. If like me, you believe in infinite space with infinite other universes, then our universe can be treated as an object, and so has an outside to it.
You - "Our universe likely has a "boundless" condition. This applies to spacetime. Meaning, should you travel backwards in time, you would never, not ever, reach a "beginning".
Hawking struggled with this idea, until he finally (shortly before his death) formulated a mathematical explanation for this involving imaginary time."
IMO, when you say- "likely has a "boundless" condition", you've quoted a fringe universe theory, Wikipedia has it under a list of 'speculative hypotheses'. I find any theory, which assumes our universe is the be-all and end-all of everything shortsighted. Picture the initial contents of our big bang just sitting on its own in an infinite void, ridiculous to me!
In Hawking's case, (I might have misunderstood) its been sitting there an infinite time and then decided to explode 13.8 billion years ago, even more ridiculous (to me).
Much more about infinite numbers of universes and existence in my book on Amazon
Steady State of The Infinite: Time Free will Randomness Cause and effect Information and order Black holes Big bang