Are you saying you are exporting a Spine project saved with Spine 2.1.12 using Spine 3.4.02? That may be a reason why width/height are zero. In general, relying on width/height in the JSON is not a good idea anyways, as it may not represent what you want, e.g. you have no default skin, or parts of the default skin are missing etc.
The better option is to calculate the Skeleton's bounding box at runtime, using Skeleton#getBounds() (or the equivalent) of your preferred runtime.
