This answer to my last question suggested, rightly, that there are targets dangerous, and large scale, enough that there is neither overkill nor spillover damage when it comes to attacking a planetary target. This is usually a variation on grey goo, sometimes organic and sometimes technological but almost always able to survive in space as well as on an otherwise habitable world.
We're pretty sure that if something hits some planets hard enough pieces come off and go into interplanetary space. Mars is relatively small and airless and I'm interested in a habitable, Earth-like, world; we're also pretty sure that the Chicxulub blast did put out ejecta that got at least as far as the moon. That blast was caused by an object estimated at 60km across well above the size generally depicted for kinetic weapons carried on or used by star ships.
So the question is, assuming a hardened pathogen (one that can survive both the impact itself and an extended trip through space), what is the upper limit on the magnitude of a single impact during a kinetic orbital bombardment to ensure that such material is not spread beyond the world being bombarded, is impactor speed or size more of a factor or is it purely a matter of the total magnitude of the blast produced?
I'm assuming that any ejecta that reach escape velocity, just over 11kms-1, are going to be a potential vector.