I would guess it depends on position of missile detonation in relation to target position, is it infront, behind, or to the side/up/down.

If it is behind, I would go with farokh's 5m because of the shockwave, but depending on target speed this could do little damage.
If it is in front, then 12m, as mdore suggested shrapnel spreading.
If it is to the side than 5m again, as 12m shot would have shrapnel fly behind a target (taking 60 degree shrapnel cone)

Of course the more I typed the more I felt it has nothing to do with the warhead, maybe it is a trick question. Then maybe Im just getting paranoid. smile