Too big because it is out of touch with its citizens, and a UBI would lead to a national ID. Look at China for what can happen from that. Also too large because the cost of living varies from place to place, so no one amount of UBI can fit every citizen.

At the same time it's too small because Americans are already far better off than most of the world, so if anyone really needs UBI it should be the poorer per capita countries, funded by the richest in the world. In other words UBI for Americans only misses the point.