Line up three containers that differ obviously in size. Name them as you touch each: big, middle, small.
Muddle them and ask your child to find the big one. Then the small one. Leave the middle one until they are confident with the ends.
When those are easy, nest them together and let your child take them apart and rebuild the stack.
Size is relative, not fixed — the middle cup is both big and small depending on what it sits beside. Grasping that a label can change with context is a real conceptual leap and the entry point to ordering and comparison.