I spent nearly 5 minutes trying to find a map of undeveloped land in the USA, but I failed. I'm pretty sure I posted it for you in the past neverdem. The US has lots and lots of land and resources. I do not know what our optimum population is but we are no where near it today. As mankind improves the way it uses resources the optimum population will increase further. 1 Billion sounds about right for the amount of land and resources we have today. In 50 years as new technologies increase energy production and construction capabilities that number may double. I hope this answers your question.
Many of our hinterlands are depopulating, large urban areas are already congested and/or blighted, surrounded by boring, expansive suburbs, of which many of them don't have enough water. Importing more bodies sounds like just the ticket. Definitely, that's the ticket!