Six boxes, each holding the same thing twice, once big and once small: two apples, two fish, two houses, two balloons, two cupcakes and two rain boots. A banner on each box says BIG or SMALL, and the child colors the one it asks for.
The banners switch back and forth, so a child cannot settle into coloring the big one every time. The big picture also moves from left to right between boxes, which means they have to actually compare the pair rather than pick a side.
A grown-up may need to read the first couple of banners aloud. BIG and SMALL look quite different on the page, so most children start telling them apart by the end.