I've played with one set that's 806666423 x 448818830 pixels, and OpenSeadragon is fine with it. That one may not count, exactly, because it's what's called a sparse pyramid… Some areas go all the way down to full resolution and others don't.
That one is a science visualization, so it's generated rather than being photographic.
Of course photo stitches keep getting bigger and bigger.
And satellite, as you suggest. I wonder what the full resolution of a complete satellite map of the world is.
I haven't heard of anyone getting to that point, though.
The symptoms would be zooming and panning would no longer be smooth but would instead pop from place to place.