Well, the habitable regions of it. Owing to the geographical set up, it is not a world in the sense of a planet, but the bird does not fly around the entire universe. Only the parts in the middle are habitable, and where the bird flies marks the very end of where humans (or human-like beings) can survive without constant magical protection.
There are birds that migrate from the Arctic to the Antarctic every year, so birds can fly pretty far pretty fast. I could make the world substantially bigger than ours, but then -- people have to move about in this story. There's a kingdom in the center of the world that has to contain a fair amount of the world -- half, perhaps. There's magic to let you move, the kingdom having a fair chunk of it, but there's also a fair amount of travel by foot. And I certainly don't want gates to do it. You can move as fast as you please, but you hit every point along the way.
Decisions, decisions. . .
Perhaps it flies about like a songbird doing an ordinary day, not like a migration, which would let it do so at leisure -- it merely circumscribes the world in a leisurely manner.
