What is the smallest seed, according to Jesus?

Mustard seeds

(He was wrong about that. Orchid seeds are the smallest.)
Another parable put he forth unto them, saying, The kingdom of heaven is like to a grain of mustard seed, which a man took, and sowed in his field:

Which indeed is the least of all seeds: but when it is grown, it is the greatest among herbs, and becometh a tree, so that the birds of the air come and lodge in the branches thereof. Matthew 13:21-22