Read this very carefully.
The question in the OP is inherently flawed. It makes a significant mistake in assuming that the motion of galaxies in 3d can in any way shape or form be used to determine their previous position in 4d.
Repeat: The motion of a galaxy that we see is completely unrelated to its “original” position in 4d space near the time of the Big Bang.
Poster after poster to this thread keeps making this fundamental mistake.
Go back to the Bubble Universe analogy. Take the galaxies on the surface of the bubble, draw vectors for each galaxy on the surface of the bubble that points in the opposite direction of its current motion on the surface of the bubble.
All of these vectors are tangent to the bubble and therefore not a single one passes thru the interior of the bubble let alone passes near the center of the bubble.
In 3d, hardly any pairs of these vectors would come close to intersecting.
And from the point of view of observers in the bubble universe, these vectors correspond to arcs of great circles. The complete great circles themselves would not all interesect at two points (like lines of longitude on the earth). They would appear to be like an extremely messy ball of twine. Intersections all over the place.
Knowing “exactly” these great circle arcs is utterly useless in finding the center of the bubble if your universe is 2d.
So, you measure exactly the proper motion of some far off galaxy. You get a vector in 3d. This vector is orthogonal to the vector pointing from the galaxy to the 4d BB start point. All our vectors in 3d are orthongal to the 4th dimensional axis.
We cannot point to anyplace in the universe and say: “That’s where it all started.” It’s orthogonal to anything we see see, measure or calculate.
A bubble being cannot point to the center of its bubble. Anything it points to is always at a right angle to the center of the bubble.
Running the motion of galaxies backwards, they all converge not so much because of their motion in 3d but because the space between them is getting smaller. The great circle arcs on the surface of the bubble get closer and closer together.
BTW: Asking a question about the size of the universe in the Big Bang model but not allowing the assumption of the size once being zero is like asking about the diameter of the earth but not allowing the assumption that it is round and not flat. The Big Bang models (standard, inflationary, etc.) all start with a size 0 universe.