Borg territory is very large, their space spans thousands of star systems ( Captain Kathryn Janeway ), and continues to expand each time they assimilate a new species.     Although Borg territory is huge, it is still small against the scale of the entire galaxy, and there are other military empires which are larger, but the Borg do not expand as other species do, they only expand their borders when they decide to assimilate a new race.

This star chart shows where in the galaxy Borg space is located.