The phrase "ex pede Herculem" is literally from the foot Hercules. The meaning is that from the foot we can deduce the size of Hercules. There is a Wikipedia article about this phrase.
The idea is attributed to the Greek philosopher Pythagoras (of right angled triangles fame). In the modern world, it is an essential tool in paleontology, where the size of long extinct animals is estimated from measuring a single bone.
ex pronoun with ablative from, out of
pede ablative singular of pes, pedis m noun a foot
