To take something that someone says with a grain of salt means that you should not necessarily believe everything he/she tells you.
I have never heard this phrase before, so I don't think it's an idiom. You can't have a herd of grain - ask the person what they actually said.
to make a bad thing worse
It means "with a grain of salt."
Don't take it too seriously.
And Take It with a Grain of Salt was created in 2002.
Depends how big the grain of salt is.
No, Salt is a mineral composed of Na and Cl ions. A grains is usually a single seed of a cereal. A grain of salt, may mean a single small piece of salt - and taken with a grain of salt is a figure of speech meaning to be skeptical.
A grain can be a tiny piece of rock, yes. A grain can also be a grain of salt, meaning one 'pellet' of salt.
The expression "I took it with a grain of salt" meant "I didn't believe it".
What she says with a grain of salt
One piece of sugar is a grain, which is also the same for salt. E.g., a grain of sugar or a grain of salt.
Large grain salt