Thank you, Andrew!
I enabled the bounds in view port, and can clearly see that the smaller of the spawned objects have small bounds, even relative to their own size (despite the fact that they are all scaled to 1,1,1 - I am assuming that being on flat planes with no thickness must be limiting the bound size, but that is not an issue.)
When I drag and drop the mesh into the view port, I can see and adjust the boundscale as per your last image. And it had exactly the effect I wanted, and that one small pieces shadow remained visible long past the rest of the meshes. Perfect!
However, I cant see a way of setting the boundscale in the content browser - so it will be applied to the meshes within the blueprints prior to spawning.
What is the best way of achieving this?
Thanks again Andrew, I really appreciate your help.