Reference for Niddah 37:28

(דברים יז, ח) כי יפלא ממך דבר למשפט בין דם לדם בין דם טהור לדם טמא

and it must be two beans in length and one bean in breadth.<span class="x" onmousemove="('comment',' Neg. Xli, 3; so that each stone is covered by leprosy of the size of one bean by one bean, which is the minimum required for effecting uncleanness. ');">²³</span>

Jastrow