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It is said:
The Midrash in Bereishit Raba describes this bone as a minuscule bone from the spine, about the size of a single barley grain, and almost cubic in shape.
See also this article on Wikipedia.
Where is it mentioned exactly, that the luz bone has the size of a single barley grain?
I did not find the particulair Midrash in Bereishis Rabbah. But, the Midrash in Koheles Rabbah (the Midrash on Ecclessiastes) explains that the luz bone is the nut-sized bone at the top of the spinal column.
“The almond tree will blossom” – Rabbi Levi said: This is the nut-sized bone at the top of the spinal column. Hadrian, may his bones be crushed and his name expunged, asked Rabbi Yehoshua ben Ḥananya; he said to him: ‘From where does a person blossom in the future?’ He said to him: ‘From the nut-sized bone at the top of the spinal column.’ [Hadrian] said to him: ‘Show me.’ What did [Rabbi Yehoshua] do? He brought him the nut-sized bone at the top of the spinal column. He placed it in water, but it did not dissolve; in fire, but it did not burn; in a mill, but it was not ground. He placed in on an anvil and struck it with a hammer. The anvil split, the hammer split, but it was to no avail.
