What is the largest single LEGO piece?
The &quot;Extra Large&quot; grey baseplate is 15&quot;/38cm square, with 48 studs to a side.
Alternatively, the moulded castle baseplate had a greater volume (with height), and a base size of 32x48 studs and 6 bricks high (15&quot;x10&quot;/38x25cm) (in other colours as well):

Tim offered up the following, also with a base size of 32x48 studs, 6 bricks high (15&quot;x10&quot;/38x25cm) (it came in a few other colours too):

Peeron also lists a 48x48 Raised with Staircases and Pits:
Peeron.com
As well as a very old &quot;Giant Base Plate&quot; and a &quot;Road 3-way Curve with Gray Pattern&quot;, both at 50x50 studs.
Joubarc suggested the &quot;Boat Hull Floating 74 x 18 x 7&quot;:
Peeron.com
Stretching the definition of &quot;Single Piece&quot; somewhat, the &quot;Electric Technic Control Centre with Yellow and Red Buttons&quot; from the Technic Control Center was a beast of a part:
Peeron.com

Made this CW to allow others to add their contenders