In one of the Poirot books he mentions that though clues are often small he once found a four-foot long one.
Unfortunately I can't remember in which book he mentions this.
Does anyone know which book he did find the four-foot long clue in?
He found it in The Murder on the Links, and he made that statement in Lord Edgware Dies.
In chapter 7 of Lord Edgware Dies, Poirot mentions this:
‘I found a clue once,’ said Poirot dreamily. ‘But since it was four feet long instead of four centimetres no one would believe in it.’
This is in reference to The Murder on the Links, where a 4 (or two, see below) foot pipe is used to disfigure a victim's face to make it unrecognizable.
On a side note, in The Murder on the Links, the pipe is said to be two feet long.
'Eh bien! I also have found something! A piece of lead-piping.'
'Nonsense, Poirot. You know very well that's got nothing to do with it. I meant little things—traces that may lead us infallibly to the murderers.'
'Mon ami, a clue of two feet long is every bit as valuable as one measuring two millimetres!
(also in chapter 7)
This seems to just be a minor discrepancy, and I don't think the difference really matters.