If I understand correctly, Mexico today is just under 2 million
km^2. With independence in 1821, it lost just over 500,000 km^2 in
Central America. Before that, as New Spain, it lost 2.1 million km^2 in
the Napoleonic Wars eventually to the US in 1803. If this is correct, it
was roughly 4.6 million km^2 prior to the Napoleonic Wars. This does
not count the loss of Florida and islands in the Pacific and the
Caribbean, which I think are smaller than these other territories.