What is the type of mustard plant mentioned in the Bible?

In the context of the Galilean farmers, the mustard seed was the tiniest of the seeds sown in the first century C.E. In ancient Jewish writings, it was used as a figure of speech for the very smallest measure of size.

Although several kinds of that plant are cultivated in Israel, very probably, he referred to the black mustard plant (Brassica nigra or Sinapis nigra), which is only about 0.039 to 0.063 inch [1 to 1.6 mm] in diameter. 

The average mustard plant is from 6 to 20 feet tall, with a 20 foot spread, although some can grow up to 30 feet.  Growth seems to be more a matter of proper cultivation than any particular species.

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The figurative species.