Currently it is not possible to animate the scale of a bone in either avatars or animesh objects.
This seems to be the case mostly to avoid compatibility with avatar shapes as they use bone scale but it causes several annoyances, especially with animesh objects.
Since animesh objects aren't influenced by their object size it seems to be entirely impossible to scale them inworld and scaling the root bone in the rig is not possible either. This means to create an animesh object with multiple different sizes we need to upload separate mesh objects. (And each with their own set of animations provided they aren't affecting purely bone rotation, but also location.) Which creates a lot of extra work and cost.
Perhaps more subjective:
Animating scale is
incredibly
important for convincing animation that doesn't feel stiff. Especially for more action loaded stylized animations. Squash and Stretch is the first principle of animation for a reason. As things are now, unfortunately it is not applicable for SL.