In ANH, we learn several things about the territorial claims of the Empire. First and foremost, the Empire controls a million star systems, making up part of a modest sized galaxy far away from our own. These systems are divided into two groups, the inner systems and the outer systems. Outer systems, such as Tattooine, are on the fringe end of nowhere, and receive little Imperial attention. It is likely that Imperial territorial claims do not include all star systems within its boundaries.
If this were so, the Empire should be roughly one thousand light years across. However, Rebels may maintain conveniently hidden bases, and the introduction speaks of a hope to "restore freedom to the galaxy," which suggests the Empire represents an oppressive force to the galaxy, which is likely to contain more than a million systems.