The ALOHA circle is that circle that contains the number of users that equals a 100% full channel. No matter where you live or how dense or sparse the activity, the 1200 baud channel can only handle a certain maximum number of packets before reliability to others falls off drastically. In LA, the range of that circle may only be 15 miles. In Wyoming, it might be 100 miles. This is *independent* of topology. The size is only limited by the number of stations and their statistical transmit rates. If you are sending packets beyond the range of the ALOHA circle, you are being inconsiderate and adding QRM to others.
See http://www.ew.usna.edu/~bruninga/aprs/ALOHAcir.txt for full description.