Hello, what you're seeing is not the root size, but the skeleton size. The root starts after that, in the next parenthesis, and only its position is noted because it's a bone without length.
A skeleton with those two values can happen if, for example, you exported the json while hiding parts of the skeleton, or the whole skeleton itself (I've seen it happen more often when a project had multiple skeletons, and some of them were hidden for convenience).
This was solved in beta but it might still happen on older versions of Spine Width and length are set to 0 if skeleton is not visible on export · #335 · EsotericSoftware/spine-editor