You can use Skeleton.GetBounds() to get the bounding rectangle for the current pose of the skeleton:
https://github.com/EsotericSoftware/spine-runtimes/blob/4.1/spine-csharp/src/Skeleton.cs#L679
This will use all the attachments currently visible on the skeleton. If instead your skeleton has explicit bounding box attachments, you can use the SkeletonBounds class:
https://github.com/EsotericSoftware/spine-runtimes/blob/4.1/spine-csharp/src/SkeletonBounds.cs
