After the fall of the city of Asgard, the Asgardians rebuilt it where it had fallen, and incorporated the remains of New York City into it, creating New Asgard. It is unknown how large it actually is, but it encompasses at least from what used to be New York State to Massachusetts.
Alternate Universe Versions
On Earth-98101 New Asgard was established in the place of what was once New York City after the Asgardians were trapped in Midgard when the Bifrost and Asgard were reported destroyed. This was in fact a ruse by Loki, but Odin knew it and allowed Loki to keep the Old Asgard for himself anyway, for the Kingdom was no good to him with the subjects to manipulate.