Fallout 4 is set in Boston, Massachusetts. Below you can see screenshots of the map taken via Pip-Boy while inside Power Armor (making the map larger on-screen) then stitched together to give you a good look at the map.
An estimated length key is included on the bottom right of the image to give you an idea of the size of the map, which using my best estimate is around 29,160,000 square feet (UPDATE: Previous calculation was missing one variable. I apologize for this).
Be sure to click the image for full size. To see all the graphics settings on the PC version read here. We also have a full list of every perk in the game.