Reference for Chagigah 21:15

[He is impure] only when he touches a part [that is big enough] it is like the entirety. The sages have calculated that that [is the size of] a lentil, for a snail at first is about the size of a lentil. R. Yose in the name of R. Yehudah says, As the tail of a lizard.

Jastrow