Extension refers to spread in one or more dimensions, such as length, breadth or depth. Such extension allows distinctions between portions or hypothetical regions, connecting it with questions of magnitude and boundary.
If God were extended in this sense, one would ask how far that extension reaches, in which direction, and how the dimensions relate. These are questions about material measure.
Conclusion: Unlimited power should not be recast as unlimited physical size. Power is a perfection; size is a measurable bodily quantity. The two are categorically different.