I'm have a problem understanding how to connect the fact that the universe is infinite, flat, had any particular size...

This may be confusing some concepts:

- finite (ie does not go on forever) and bounded (ie has a distinct boundary).

- flat (ie obeys Euclidean geometry) and curved (ie does not obey Euclidean geometry, if you measured it accurately enough, on large enough scales)

A 2-dimensional analogy may be the surface of the Earth, which has a finite area, but has no distinct boundary, so if you continue in one direction, you eventually reach your starting point. It is curved on the local scale (hills and valleys)

*and* on the global scale, but you would have trouble proving this with parallel lines drawn on a sheet of paper.

A hint of this is contained in the dismissive

consider curved space where going enough forward will eventually bring you backwards but that's irrelevant

We can't measure the universe well enough, or see past the Big Bang to determine if we would arrive back at our current location, if we left at some time in the past.

The discovery of Dark Energy suggests that if we left

*now* with an ion drive spaceship, we would

*not* arrive back at our starting point, because the universe would have expanded faster than we could travel.

All our measures suggest that while space is curved on the scale of solar systems & galaxies, it is flat on larger scales. But perhaps we are not measuring on a

*large enough* scale?