One of the many problems with this question is that it precludes a reliance on physics. If God exists and if God is omnipotent why would he be subject to the laws of physics in the first place? If God can create matter from nothing he clearly operates outside the bounds of physics as we understand it. Then there is the question of size itself. Size is not what gives an object weight. That's a function of mass. That's why you can have a cubic foot of pumice that most people can lift without issue compared to a cubic foot of iron which would be very difficult to lift. Furthermore, lifting an object would require more than one object. When we lift something we are countering the gravity between that object and the earth. If God were making an object of such incredible mass so as to be immovable he would need to in fact make two such objects. As far as philosophical quandaries go, this one is not the best.