What factor primarily determines the size of an orbit?

The size of an orbit is primarily determined by the speed of the object in that orbit. According to the principles of orbital mechanics, the gravitational force acting on an orbiting object and its tangential speed are interconnected. When an object moves at sufficient speed, it can maintain a stable orbit around a central mass, like a planet or a star. If the speed is too slow, the object may fall into the central mass; if it's too fast, it could escape the gravitational pull altogether.

The relationship between speed and the radius of the orbit can be described using Kepler's laws of planetary motion and Newton's law of gravitation. For example, a planet closer to the sun must travel faster to balance the stronger gravitational pull compared to a planet farther away. Consequently, the orbital speed dictates how large or small the orbit can be, as the gravitational forces and centripetal forces must be balanced for a stable orbit to exist.

While the distance from the sun does influence gravitational strength, it is the object's speed that ultimately determines whether and how it will orbit. The other factors, such as the mass of the orbiting object and the shape of the orbit, play roles in certain contexts but are not the primary determinants of the orbit's size