There are 172 million acres of land in Texas. That means you could give every American man woman and child a half an acre of land to themselves in Texas, with land to spare. The US is really big.
You could also (in theory) put every human on Earth in Texas with a population density of only 26,000 people per square mile, about the same density as New York City.
If it's an equilateral square pyramid, the volume would be about a quarter of a cubic kilometer. If we assume humans have an average mass of 65 kg and the same density as water, that gives us room for just under 4 billion humans.