Is this a valid argument against an omnipotent being? Discuss.
Nah.
I'm not a religious guy by any means, but even if I was this argument gives off the same feeling as the ever popular "if a tree falls" question...
It's a paradoxical question, there is little more to it than that. Even if a god exists, there is no real way to measure what the extent of it's power would be.
...not to mention that if the god created the earth, and by "lifting" we're assuming on earth, I would also assume that god is stronger than the force of gravity and could probably lift anything.
...unless of course god manipulated the downward force of gravity >.>
In that case, would the size of the object really matter? No.
Rambling semantics aside, it isn't a good argument because it doesn't disprove anything, the only thing it proves is that it's impossible for humans to comprehend a solution to such a paradox.