While there are multiple sizes of instructions and lack of database with dimensions my answer is based on personal observation.
Looking at the stash of my instructions from multiple sets (including several largest Technic models) I can state that the largest dimension I have is of A4 paper size, which is 210 x 297 mm. I'd like to note that this is for ...
If you mean specifically Lego bricks, I'm pretty sure the 1x16 was/is the longest one made. Outside of the 1x? sized bricks, the longest plain shaped brick that I know of is part #30072 or #47122 at 24 studs long. I don't know about other brands though.
Interesting related post:
What is the largest single LEGO piece?
No restrictions on the materials.
Well then, probably a component to the old wooden furniture LEGO used to produce, like these:
5003483 - 3-Seat Playtable Creative Play Station Center Pack Duplo
Or the similarly designed 4509g - 3-Seat Playtable, 5004063 - 3-Seat Playtable, or 5003468 - 3-Seat Playtable Creative Play Station Center Pack.
IF-LTW2C - ...