Thanks for your responses.

I've got a confirmation from Smith Micro that Firefly does not handle depth maps on small objects.  If you put two small objects in the scene, unless they are very close, then it works.  It seems to be the target area that Firefly tries to map that matters.  Two objects makes the map area larger.

In the default light set up the threshold seems to be about 8 1/2 inches.  And this is with Infinite Lights.  Change to Spots and the object renders fine.  I haven't tried to move the spots way out to see if distance is a factor.

You can test this by using Poser primitives like the Hi-Res Ball.  It loads at about 10 inches so if you scale it down to 60% you wind up with bad shadow map.  Scale it back up to 100% and it renders fine.  Also, use two spheres, scale them way down, 10-20%, put them close together and the shadow map is bad.  Move them apart far enough and they render OK.

For practical purposes, it really doesn't matter much.  How often does anyone want a render of one small object?  I was just making a temp thumbnail for a prop for my library when I stumbled across this limitation and panicked.  Now that I know it isn't my mesh that's the problem I'll just work around it.

Thanks again everyone.