I have a 16x16 stud clear base plate. Where is it from?
Any of these three sets

(AKA:
6162-1 A World of LEGO Mosaic 4 in 1
6163-1 A World of LEGO Mosaic 9 in 1
852676 Travel Game - Multi Game Pack 9-in-1)

Probably.

Bricklink hint: just search for a size, like "16 x 16". "3x3 plate" is fun too.