What do you mean by "sprite size"? Do you mean the height and width on screen?
And what do you mean by "each time when the animation is running"? Do you mean every update?
Images are called "attachments". You can access this in a slot's slot.Attachment
If it's a mesh, it's a MeshAttachment. And it only contains vertices.
You can also get world vertices from a RegionAttachment based on how its four corners are positioned by the skeleton.
You can iterate through all those vertices to find the min and max values, which can then be used to define a Rect.