The population of NCR during Fallout 2 was 700,000 and that was before NCR took control of all of California and into Mexico and Nevada. Also close to 30 years before Fallout New Vegas. By now I would not be suprised if NCR has a population of at leastt 3 million. Much better living conditions also adds to the population boom. Like Arcade said "with better medican people live longer, funny how that works." Caesar's legion possibly close to a million in there lands.
Seeing as how California had the greatest population in the united states and seems to have advanced the most since the great war. I would think that the rest of America has about 2 million in total.
"In two brief hours most of the planet was reduced to cinders" Fallout intro
"The Earth was nearly wiped clean of life." Fallout 2 intro
We don't know what the rest of the world is like but we know that the rest of the world was nuked to hell like America was. It has taken America 200 years to get a population of just under 6 million people. I would have to say the rest of the worlds population would be no more then 20 to 30 million people.
My math svcks but I think the math would go something like this.
Take the population of America before and after the great war and find the percentage
Then take the population of the world before the great war and reduce it by the same percentage as above.