The barnacle! At least in the invertebrate world. Sort of a boring answer I suppose. Until you learn that barnacles can alter both their penis size AND shape.
The barnacle penis is eight times the length of its body. Its shape depends on the environment in which the barnacle resides (eg. barnacles in stronger waters will have a more stout penis). Barnacles are hermaphrodites (both male and female genitalia) but prefer to mate with other individuals. Since they lead a sedentary life, finding another barnacle to mate with can prove difficult. Their penes (yes that is an acceptable plural form of the word penis), therefore, have to be long enough to reach a mate. What most men wouldn’t give right now to be a barnacle…
Largest penis size in proportion to body size amongst vertebrates belongs to the Argentine Blue-bill duck.
While the penis is normally about as long as half of the duck’s body, it can be as long as the duck itself. Lucky little females, eh? (Or perhaps terror-stricken…)
The theory behind the length is based upon the competitive nature that comes with these promiscuous birds. Their long, corkscrew shaped genitalia can then act as a method of removing the sperm of competing males. Most birds don’t have external genitalia, but waterfowl do to help with more accurate fertilization in their water based environment.