Which one makes things become bigger? I learned that one first and then later figured out the one that makes the mixed image smaller (cross eyed I think?). Now I cannot do the big one anymore.

For me, what's difficult is holding my right eye closed without my left eye drifting to look at my nose. My right eye's good, I can move it and focus on anything within my (now peripheral-limited) view... but the left is wonky. I think I learned how to wink (and hold it) with the right really early, by age 3 or 4, but the other side I never tried until I was pre-teen... some sort of muscle atrophy?

You can also tell if your head's level, just by crossing your eyes. If the two images are diagonal to each other, then your eyes/head aren't level. I have no idea what the possible use for that would be.