Short answers, pulled from the story.
The performance space of the Roman circus was normally an oblong rectangle. Two linear sections of race track ran parallel to each other with a median strip called the spina dividing the course down its center.
Most circuses were built during the 400 years between 200 BC and 200 AD. The Circus Maximus in Italy may have existed in some form from as early as 500 BC before transforming into a massive stone complex over many generations.
Estimates for the Circus Maximus place capacity around 150,000 people. Some estimates push this figure up to 250,000 spectators compared to smaller venues like the Gerasa circus which held around 15,000 people.
Circuses do not appear to have been constructed with any special compass orientation. Well-identified examples show their round ends facing various directions such as north at Gerasa or southeast at the Circus Maximus.
The best preserved circuses include sites across Spain, Italy, and Turkey including Emerita Augusta in Mérida and Caesarea Maritima on the coast of Israel. The Hippodrome of Constantinople in Istanbul measured approximately 450 meters in length but virtually none of the structure remains visible today.