How large could the following empires, which were some of the largest empires the world has ever seen, even in OTL, conceivably have gotten? (Not all in the same TL, just hypothetically. And you don't have to answer all of them.)
The Ottoman Empire
The Mongol Empire
The Caliphate (any of them)
The Timurid Empire
The Eastern Roman Empire
The British Empire
The Spanish Empire
The Russian Empire (or the USSR)
United States of America
Mexican Empire
Or the Roman Empire, but that's been talked about so much and people get really fierce about it.