For a collision to be elastic, by the usual definition, no internal degrees of freedom of the colliding bodies can be excited/de-excited by the collision.
The internal degrees of freedom that might change in a collision include vibration, rotation (although some might argue about this), electron orbitals, electron spin, nuclear spin, etc. etc. etc.
For macroscopic objects, the density of states for all these parameters is so high that in practice it is astronomically unlikely that a TRULY inelastic collision will ever occur. This is true even if the bodies are spherical, and have the same shock wave round-trip times, etc. etc. The energy "lost" in a collision may be so small that it is extremely difficult to measure, but any collision of macroscopic objects is sure to be inelastic.
The situation is very different if the colliding particles are single atoms. There, the density of internal states is so low (and coupling to various degrees of freedom so weak in the scattering) that it can be thousands of times more likely to have an elastic collision than an inelastic one.
Diatomic molecules are appreciably worse than atoms; it is relatively easy to excite/de-excite rotational motion of the molecule in a collision. Inelastic collisions are about as common as elastic collisions.
Where's the tipping point? By the time you've got something as big as, say, a virus, I'm sure it's extremely improbable to have elastic collisions. But for something like a buckyball? I have no idea.