Why mustard seed ?

Mustard seeds of the various mustard plants are among the smallest of seeds. The seeds are about 3mm in diameter, and may be colored from yellowish white to black. Some explain In term of Inch of Mustard seeds are about one-sixteenth of an inch in diameter.

Today, we know that there are seeds even smaller than the mustard seed. For instance, the orchid seed is so small and fine that it's almost dust-like. Certain epiphytic orchids of the tropical rain forest produce the world's smallest seeds, up to 35 million per ounce. One seed weighs about one 35 millionths of an ounce (1/35,000,000) or 0.81 micrograms. Some seeds are only about 1/300th of an inch long (85 micrometers).

Now, was Jesus wrong about the mustard seed?

Sensible Solution:
Please note that Jesus was not comparing the mustard seed to all other seeds in the world, but to seeds that a local, Palestinian farmer might have "sowed in his field," i.e., a key qualifying phrase in verse 31. And it's absolutely true that the black mustard seed (Brassica nigra = Sinapis nigra) was the smallest seed ever sown by a first-century farmer in that part of the world.

It's also true, as many modern-day encyclopedias will tell you, that the black mustard see in Israel will typically grow to heights of 3.7 meters, or twelve (12) feet) -- plenty large enough to hold a bird nest.

It's important to remember that the Bible often uses everyday terminology in order to communicate simple truth. Even today, we might refer to a "sunset" when, technically, scientifically, we know that the sun never actually 'sets,' i.e., it's the earth that revolves.