From what I understand, there’s only one way to scale blueprint actors, and that’s on a per component (usually static mesh) basis. I’m not sure what the correct name would be for scaling entire blueprints, so I’m just going to refer to it as “global scaling”.
If you try to construct a complex blueprint actor from several different static mesh components, offset or rotate some of the meshes, now the blueprint actor cannot be scaled from the main UE4 viewports because the blueprint actor will scale in a really inconvenient way. All of the static mesh components scale independently, and in a very awkward way.
For example, lets say you want to make a doorway. The doorway may be made of several components. The frame, the inner walls, another frame… etc etc. Many pieces make up this doorway. In the process of constructing this blueprint actor, mesh components may need to be rotated or scaled, or even offset. Now, once placed into the level, you may want to scale the entire blueprint actor on a global basis, which currently isn’t possible.
This means for more complex blueprint actors, you’re better off constructing your mesh in a 3d modeling program of choice, then importing into UE4 so that it can be scaled properly in UE4. Obviously this means, more meshes, more memory usage, etc. Not exactly an efficient way to do things.
Now, factor in light components for your blueprint actors. Light components may be rotated, scaled, lengthened, etc. These would have to be properly scaled if global scaling is implemented.
Hope this is considered! 
Thanks, -