Banana Slugs are the largest of all North American slugs. Most adult Ariolimax slugs are about 15 - 20 cm (six to eight inches) in length. They can reach a length of 25.4 cm (10 inces). They are the second largest slug in the world, the largest being the Limax cinereoniger of Europe, which can reach 30 cm (12 inches) in length.
Probably not the biggest, but in my garden there are a succession of fat brown slugs hiding under my heather plants which are between two and three inches long. Whenever they appear I transfer them to my bird table where they rapidly disappear ! Probably the equivalent of fillet steak in the birdie world!
Sorry to contradict you all, but the biggest slug in the world is the one I trod on earlier in bare feet whilst putting the rubbish out!! And I live in Derbyshire. Yuk!! >>>>>>>>>>> goes to find salt pot