The Northside Territory was a nation-state on the planet Sigma Iotia II. It was the largest such territory on that planet, and it was controlled by Boss Bela Okmyx. Being the largest, it was often the target of attacks by rival bosses, such as Jojo Krako of the Southside Territory. (TOS: "A Piece of the Action")
It is never revealed in the episode just how large the Northside Territory is; however, using a conventional 1930s era automobile, Kirk and Spock are able to drive back and forth between the headquarters of the North and Southside territories several times (even with Kirk's poor ability as a driver).