Relative to what it was before. It has an intrinsic size. Think of it in terms of matter density if you find it more comfortable. The density simply goes down over time; distandes between galaxies increase.

Density has the same issue. Density can only be measured against a baseline established outside of the medium being measured.

Just because you can't measure something directly doesn't mean it doesn't exist. There's plenty of things we only know only through indirect measurement.

Intrinsic expansion. Distances between objects in space grow.

That's what metric expansion of the universe is. Distances grow at speeds that are proportional to their distance from the observer.