What is meant by "spacetime" is a description of the geometry of the universe, ie. Minkowski spacetime.
Minkowski space-time is the space-time of Special Relativity. Our Universe is not Minkowskian except over small distances. Same way that the ground is not Euclidean (we live on a spheroid) but appears Euclidean over small enough distances.
That is finite in size down to Plank scale.
I think I know what you are saying here, but the Planck Scale is a scale in the same sense as the atomic scale, and refers to lengths on the order of the Planck Length (1.6 x 10
-35) We sometimes use the phrase, *THE* Planck Time, to refer to the earliest moments of the Universe following T=0, but *A* Planck time is simply the geometric length of time, 5.4 x 10
-44secs.
I think the confusion comes in when the conversations are trying to describe spacetime as infinte, when it obviously is not. However, it can expand to infinium
No, it could well be infinite in extent spatially at all times T>0 (at T=0, length has no meaning - but that doesn't mean zero-sized!), irrespective of expansion.
The Big Bang space-time (with Cosmological Constant) is finite in extent in the negative time direction, infinite in extent in the positive time direction, and finite or infinite in extent in the spatial directions. If finite, it wraps back on itself as there is no edge.