When loading JSON or binary for info or import, it gets converted internally to a Spine project. We did some tests and it's a bug that the JSON or binary editor version doesn't make it into the Spine project. We'll fix that in 4.2. Same problem with the path that should be after
The "size" is the AABB of the setup pose. Normally the editor calculates this, but it doesn't get a chance when just printing the project information. We'll fix this too!
Size: should be some lines about the bones in the skeleton. I guess you omitted those? After that more lines are printed about slots, skins, events, etc but only if the skeleton contains those items.
Hopefully you've got what you needed. At least your one skeleton came from 4.0.47, so you can use the latest 4.0 version to import the data to a Spine project, if that is your goal. You can than inspect the skeleton in the editor. You could also use Skeleton Viewer (be sure to use 4.0) to look at the skeleton data directly.