The expression "needle in a haystack" comes to mind!
I prefer sending my apprentice to find a piece of hay in a needlestack.
01 Grain = 01/5760 lb (64.799 milligrams)
pit of the gaping earth : the dirt in a circle 6 paces across recedes into the ground leaving a pit of 9 feet deep
that spell is in "part" as it is a part of earth (not Earth) which is a whole.
For sand, I would also take "part" better than individual
6 paces across and 9 feet deep is +1 size
So in fact for sand I would use part and not individual or group... except if you have sand in a bucket maybe...
size 2 would be 5 foot wide, 60 foot long and 10 foot deep (exemple : the earth split asunder)