What is the largest single LEGO piece ever produced? This does not include LEGO affiliates like Duplo.
It must be a purchasable block.
By largest, I mean largest by Volume/Weight of Plastic
The "Extra Large" grey baseplate is 15"/38cm square, with 48 studs to a side.
Alternatively, the moulded castle baseplate had a greater volume (with hight), and a base size of 32x48 studs and 6 bricks high (15"x10"/38x25cm) (in other colours as well):
Peeron also lists a 48x48 Raised with Staircases and Pits:
Joubarc suggested the "Boat Hull Floating 74 x 18 x 7":
Stretching the definition of "Single Piece" somewhat, the "Electric Technic Control Centre with Yellow and Red Buttons" from the Technic Control Center was a beast of a part:
Made this CW to allow others to add their contenders
if you're just counting the basal area, it seems to be the gray 48x48 baseplate:
If you go by Volume/Weight. This is heaviest single-element I've ever seen. It's not just plastic but it's one element made and sold by LEGO.