The answer depends on the kind of salt that it refers to. A grain of table salt has a mass of a third to two-thirds milligrams. Large salt crystals, like those used in salt grinders (or for margaritas) have a mass of a milligram or more. The question is ambiguous: like "how long is a piece of string?"
What she says with a grain of salt
Id say a milligram of salt equals a milligram. A grain of salt is rediculously small, way smaller than a milligram, about 100 Micrograms
a grain of sugar
The mass of one grain of rice is 25 milligram or .025 grams. There are about 29,000 rice grains in approximately one pound of long-grain white rice.
something similar to one grain of table salt.
milligrams
It depends on whether you wish to measure its length, its volume or its mass.
A microgram.
And Take It with a Grain of Salt was created in 2002.
Depends how big the grain of salt is.
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
With a Grain of Salt - 1913 was released on: USA: 14 March 1913
A Grain of Salt - 2008 is rated/received certificates of: USA:R (original rating)