It would be simpler and more easily visualised for Him to just increase the size of the current universe and create a small pebble in it, so that He could move the previously-universe-sized rock in relation to the smaller pebble.
However, that's not the point. I was just showing that God could create a rock so big that He could not move it (at least, immediately) without infringing on His omnipotence, using just a semantic argument.
Point is, while it can be an amusing question, it's hardly a 'disproof of God', even without having to resort to "well, because!" arguments.