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 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.
if you're just counting the basal area, it seems to be the gray 48x48 baseplate: