Roughly speaking, what would be the area of Middle-Earth? Would it be similar to that of Europe, or France or Eurasia?
And, when speaking of the population, I mean Men and Elves and Hobbits, and all more-or-less human-resembling races.
Much of Middle-Earth has a definite European feel to it, and many of the characteristics of the races can be seen to have been based off of different areas around Europe. With the Shire in particular, being referenced as in the
"the North-West of the Old World, east of the Sea"
And in Letter 190 from The Letters of J.R.R. Tolkien he stated:
'The Shire' is based on rural England and not any other country in the world
The only other point of comparison Tolkien gives is in one of his letters where he references the equivalent latitudes of a few locations in Middle-Earth on the modern map:
The action of the story takes place in the North-west of 'Middle-earth', equivalent in latitude to the coastlands of Europe and the north shores of the Mediterranean. ... If Hobbiton and Rivendell are taken (as intended) to be at about the latitude of Oxford, then Minas Tirith, 600 miles south, is at about the latitude of Florence. The Mouths of Anduin and the ancient city of Pelargir are at about the latitude of ancient Troy
As for the break down of the populations, which is what my answer will focus on as it is less conjecture, there is a reason that so much of it is humanoid. The origins of all of the Free Peoples of Middle-Earth stem from or fairly closely involve Eru Ilùvatar, the single creator. He created Men and Elves, from whom more races came to be, such as Hobbits. The Ents were created by Eru as well, but in order to please Yavanna, and the Dwarves were created by Aulë but needed to receive his blessing in order to gain free will. Even the Orcs and Trolls, while created by Melkor as a twisted mockery of the Free Peoples, were based closely on the humanoid races that had been created previously.
While it is possible to trace characteristics of many of the Free Peoples back to different areas of Europe, the Orcs can be seen as more Mongoloid
The Orcs are definitely stated to be corruptions of the 'human' form seen in Elves and Men. They are (or were) squat, broad, flat-nosed, sallow-skinned, with wide mouths and slant eyes: in fact degraded and repulsive versions of the (to Europeans) least lovely Mongol-types
Middle-Earth is definitely a European-like landscape for the most part and there are definitely some places beyond the black gates that no one has explored. Some of it would like like Eurasia, I'm certain.
In Middle-Earth, there are quite few different races of humanoids, but it is important to note that greenskins, id est, goblins and orcs, are all essentially the same race. There are different types of elves and different types of dwarves.
The main population of Middle-Earth changes, depending on the era, but toward the end of the series, man becomes the predominant race. There's actually a rant from Gimli in one of the movies that was a stab at C.S. Lewis about how dwarves are born. (In The Chronicles of Narnia, dwarves are born from the earth and not of dwarf men and women as they are in Lord of the Rings.) Anyhow, Lord of the Rings was the inspiration for many of the races of Dungeons & Dragons, as well as Dragonlance.