The size of the protected airspace while circling to land is determined by a standard radius from the ends of the runways. The radius can be found based on the aircraft’s speed and altitude.
There are five speed categories:
Category A: up to 90 knots
Category B: 91 to 120 knots
Category C: 121 to 140 knots
Category D: 141 to 165 knots
Category E: 166 knots or greater
The category can be plugged into this chart to find the radius:
The pilot should extend their radius from the end of each runway to find the protected airspace, as shown here.
References: Wikipedia, rapp.org, CFI Notebook
