Is there a way to estimate how much larger the US population would be today had the US not had a Civil War, or fought in WWI and WWII?
I know this is a ridiculous proposition, but I’m curious how wars have impacted the population of the US.
According to this official clock, the current population is just over 319 million.
As of the timestamp of this post, it’s a bit more than 316 million. You doing New Math? 
As for answering your question, you could use the yearly growth rate from say, 1800 to the present and work from there.
The two world wars affected a remarkably small percentage of the population. Almost as many people died per year from traffic accidents in the early 50s as died per year in WWII.
The Civil War took a much larger percentage of the population, about 600,000 or 2%. They were almost all relatively young men so presumably they would have contributed to the population.
This page shows the percentage growth by decade. The decade of the 1860s shows a decline, but is only about 6 percentage points lower than the next two decades. That’s because immigration to America was higher than the number of dead, and much of that immigration was spurred by the possibility of getting one of the bounties paid to soldiers. You could try adding back that extra growth and extrapolating from it, but that probably overstates how much there would really be.
WWI isn’t even a blip. WWII is lower, but that’s as much because immigration had nearly completely stopped. The revised immigration law was passed in 1924, which is why the growth slows so much before the Depression.
Immigration is such a huge factor in American population growth that making estimates merely about war dead doesn’t reflect history well.