Marvel Universe describes Asgard as:
a small planetary body that serves as home to the Norse gods such as Thor, and their ruler, Odin. It exists in another dimensional plane and is about the size of the United States.
There is currently no completed entry for Midgard, but the Asgard article addresses it briefly.
Another of these worlds that is in a different dimensional plane is the world of Midgard, or Earth.
Midgard, or “Middle Realm”, is home to humans and does not seem to be affected by the motion of the other worlds of Asgard. Jotunheim is the world of the Giants and is also on its own dimensional plane hidden from Asgard or Earth’s. The only thing that prevents passage between Midgard and Jotunheim is Jormungand, the Midgard Serpent.
What defines Midgard and its boundaries? Is Midgard just the planet Earth? Our entire Solar System? The entire Milky Way? An alternate universe?