The Blue Whale, the largest mammal on Earth, is likely the champion for this category standing at a full erection of almost sixteen feet..
"The Blue Whale’s penis is estimated at over 2 m (more than 6½ feet). Accurate measurements are difficult to take because the erect length can only be observed during mating [almost impossible to view] but wait.....In an adult male, the penis is about 5 m long (16 ft) , I bet the blue whale would go with 16ft."
http://evatt.blogspot.com/2006_04_16_archive.html
Sperm whales also have a really huge penis and are said to release over 400 gallons of sperm in a single ejaculation.
A while back National Geographic published and article on the courtship of whales and one photo was of a Sperm Whale with a full erection. It was truly awe inspiring, and disproportionately large for the animal.
Think, with no grasping appendages the whales have to makeup for that handicap somewhere. I have also heard that whale's penises are prehensile, they need to be.
As land living mammals go the African elephant has one huge member, but its minuscule compared to the big whale's.