The first answer that comes to mind would be to reduce the population by eighty percent, which would result a combined populaton of 117 million. (Or 104 million if you were to draw from 2003 populations, when the blog was original written.) But a question I asked myself--much of this are was settled and colonized for centuries before Anglo-America, but it tended to be less populated until the 1950's, that is, heavy industrialization. Considering Florida, one of the most populated states in Industrial America and the most tropical region in the continental U.S., took a major population crash, would that apply to Mexico, Central and South America?
I decided to take a two-prong approach. The first was de-urbanization. I multiplied the population by the percentage of people who did not live in big cities. It's a pretty handy trick--the U.S. is about 80% urbanized, which is also the number by which Medieval America is reduced. (It even tends to work for individual states, for the most part) However, it's not a perfect system. De-urbanizing France gives you numbers it would be at in the Middle Ages, but applying that to Great Britain gives you 12 million--more than the island can actually contain. So part two involved looking at the populations for these countries from about 1885-1890, as that would be the period the U.S. had a population comparable to this. (Which granted, is a pretty imperfect way to look at it, but I figured concentrated industrialization amongst swaths of wilderness balances itself out) For the most part, the difference between the two stats is not that significant, so I averaged the two and come up with the numbers for Medieval Latin America.
The total number is around 66 or 67 million, About a third of them border of the Gulf of Mexico and the Caribbean sea, and a third lives in a fifth of the total area--from Mexico to Colombia. This makes sense as Mexico was theorized to have as many twenty million people when Spain first arrived (I don't have it as high, but that number is conceivable), and since trade works in the dynamics of North/South as opposed to the east/west dynamics of the old world. Mexico and Central America are very much a good stopping point. Brazil takes the biggest hit--making up well over a third of Latin America's population in the Industrial era, it makes up less than a third in the medieval era. I actually ratcheted up the population a bit up to 20 million due to the vast space, and that the Parana River is pretty fertile agriculturally, but it's important not to mess with it too much