which one is bigger?

ocean is bigger, sky is smaller.

but how to measure?

you need a measuring tape, a small one for the sky and a bigger one for the ocean.

one person stands at one side, the other person stands at the other side of the ocean. they are holding measuring tape. one of the person pulls the tape.

you use a shorter measuring tape. if you want to measure the sky, ask the magic slide to sent you up. you have to be there for 2 days. one hand flapping and the other hand holding the tape.