Because they are fictional characters, their actual appearance can be interpreted by actors and illustrators. However, in Poirot's case, author Agatha Christie many times wrote that he had an "egg-shaped head" and had fabulous mustaches, though as best I know she didn't actually provide a description of them.
Since this is an art-related blog, today's post presents some interpretations of Poirot by various illustrators.
here. This predates the British television series, but his attire is similar to what it favored.
here, was an important illustrator in his day. This image and the following are from Poirot stories appearing in Collier's Magazine in the mid-1950s.
here) has pretty much nailed down Hercule Poirot's appearance for now and quite likely for many years to come.